Story highlights U.N. official says 80 of 298 victims were children

Monitors say they only spent 75 minutes at crash site

Rebels couldn't have shot down the jetliner without backing, Obama says

U.S. envoy to United Nations says pro-Russia rebels likely fired missile at plane

International monitors investigating the Malaysia Airlines crash in eastern Ukraine said Friday the team was not given full access to the site and was greeted with hostility by armed men.

"There didn't seem to be anyone really in control," Michael Bociurkiw, spokesman for the Organization for Security and Co-operation in Europe team, told CNN's Christiane Amanpour.

Armed men, apparently pro-Russia militants, loosely guarded the area but couldn't answer the monitors' questions, he said.

Bociurkiw said the group only stayed about 75 minutes and examined about 200 meters at the scene before being forced to leave. Pieces of the airplane and bodies are spread over several kilometers.

The OSCE team arrived at the crash site near Torez in a remote section of eastern Ukraine that's controlled by pro-Russian militants battling the Ukraine government.

The United States says a surface-to-air missile, possibly fired by the militants, took down Malaysia Airlines Flight 17 on Thursday as the plane traveled from Amsterdam to Kuala Lumpur in Malaysia. All 298 people on board died. U.N. Under Secretary General Feltman said 80 of the victims were children.

Photos: Photos: Reaction to MH17 Photos: Photos: Reaction to MH17 World reacts to MH17 crash – Families of crew members aboard Malaysia Airlines Flight 17 gather for a vigil Tuesday, July 22, in Kuala Lumpur, Malaysia. All 298 people aboard the passenger plane died when it was shot down Thursday, July 17, in a rebel-controlled part of eastern Ukraine. Hide Caption 1 of 28 Photos: Photos: Reaction to MH17 World reacts to MH17 crash – A woman cries July 22 during a service near the crash site. Hide Caption 2 of 28 Photos: Photos: Reaction to MH17 World reacts to MH17 crash – People in Melbourne gather to mourn the victims during a candlelight vigil at Federation Square on July 22. Hide Caption 3 of 28 Photos: Photos: Reaction to MH17 World reacts to MH17 crash – In memory of two Newcastle United fans who died in the crash, two wreaths are placed on seats July 22 at the Forsyth Barr Stadium in Dunedin, New Zealand. The soccer fans were traveling to New Zealand to watch their team play in a preseason tournament. Hide Caption 4 of 28 Photos: Photos: Reaction to MH17 World reacts to MH17 crash – A KLM employee reaches out into a sea of flowers July 22 at Schiphol Airport in Amsterdam, Netherlands. Hide Caption 5 of 28 Photos: Photos: Reaction to MH17 World reacts to MH17 crash – A flower and stuffed animal sit near the crash site on Monday, July 21. Hide Caption 6 of 28 Photos: Photos: Reaction to MH17 World reacts to MH17 crash – A woman in Kuala Lumpur attends a candlelight vigil on July 21. Hide Caption 7 of 28 Photos: Photos: Reaction to MH17 World reacts to MH17 crash – Ukrainian President Petro Poroshenko attends a flower-laying ceremony at the Dutch Embassy in Kiev, Ukraine, on July 21. Although the passengers came from all over the world, many of them were Dutch because the flight originated in Amsterdam. Hide Caption 8 of 28 Photos: Photos: Reaction to MH17 World reacts to MH17 crash – A woman places a flower during a candlelight vigil in Kuala Lumpur on July 21. Hide Caption 9 of 28 Photos: Photos: Reaction to MH17 World reacts to MH17 crash – Mourners in Eynesbury, Australia, attend a memorial service Sunday, July 20, for a family of five killed in the disaster. Hide Caption 10 of 28 Photos: Photos: Reaction to MH17 World reacts to MH17 crash – Buddhist monks in the Malaysian capital light candles during a special prayer for the victims on July 20. Hide Caption 11 of 28 Photos: Photos: Reaction to MH17 World reacts to MH17 crash – Wildflowers lie on an engine from the crashed jet on Saturday, July 19. Hide Caption 12 of 28 Photos: Photos: Reaction to MH17 World reacts to MH17 crash – Friends of Nur Shazana Mohamed, a crew member aboard the flight, take part in a special remembrance prayer at a mosque in Putrajaya, Malaysia, on July 19. Hide Caption 13 of 28 Photos: Photos: Reaction to MH17 World reacts to MH17 crash – Dutch Prime Minister Mark Rutte signs a condolence register at the Ministry of Safety and Justice in The Hague, Netherlands, on Friday, July 18. "I want to see results in the form of unimpeded access and rapid recovery," Rutte said in a press briefing. "This is now priority number one." Hide Caption 14 of 28 Photos: Photos: Reaction to MH17 World reacts to MH17 crash – People pray for the victims of Malaysia Airlines Flight 17 at a church outside Kuala Lumpur on July 18. Hide Caption 15 of 28 Photos: Photos: Reaction to MH17 World reacts to MH17 crash – A woman in Berlin places a candle at a memorial on July 18. Hide Caption 16 of 28 Photos: Photos: Reaction to MH17 World reacts to MH17 crash – People in Kiev gather to mourn the victims on July 18. Hide Caption 17 of 28 Photos: Photos: Reaction to MH17 World reacts to MH17 crash – Akmar Binti Mohd Noor, whose sister was aboard Flight 17, cries outside the family holding area at Kuala Lumpur International Airport on July 18. Hide Caption 18 of 28 Photos: Photos: Reaction to MH17 World reacts to MH17 crash – A man prays at a memorial in front of the Dutch Embassy in Kiev on July 18. Hide Caption 19 of 28 Photos: Photos: Reaction to MH17 World reacts to MH17 crash – Dutch cyclists wear a black armband in honor of the crash victims during the 13th stage of the Tour de France on July 18. Hide Caption 20 of 28 Photos: Photos: Reaction to MH17 World reacts to MH17 crash – Dutch Justice Minister Ivo Opstelten observes a moment of silence after signing a condolence book in The Hague on July 18. Hide Caption 21 of 28 Photos: Photos: Reaction to MH17 World reacts to MH17 crash – Candlelight prayers honor the victims at a church outside Kuala Lumpur on July 18. Hide Caption 22 of 28 Photos: Photos: Reaction to MH17 World reacts to MH17 crash – Floral tributes adorn the entrance to Schiphol Airport in Amsterdam. Hide Caption 23 of 28 Photos: Photos: Reaction to MH17 World reacts to MH17 crash – A Dutch flag flies at half-staff in The Hague on July 18. Hide Caption 24 of 28 Photos: Photos: Reaction to MH17 World reacts to MH17 crash – People place flowers in front of the Dutch Embassy in Moscow on July 18. Hide Caption 25 of 28 Photos: Photos: Reaction to MH17 World reacts to MH17 crash – Students attend a prayer July 18 in Central Java, Indonesia. Their teacher John Paulissen was a passenger on Flight 17. Hide Caption 26 of 28 Photos: Photos: Reaction to MH17 World reacts to MH17 crash – Russian President Vladimir Putin, center, and members of his government observe a moment of silence on Thursday, July 17. Hide Caption 27 of 28 Photos: Photos: Reaction to MH17 World reacts to MH17 crash – People pay tribute to the victims outside the Dutch Embassy in Kiev on July 17. Hide Caption 28 of 28

Photos: MH17: What they left behind Photos: MH17: What they left behind MH17: What they left behind – A birthday card found in a sunflower field near the crash site of Malaysia Airlines Flight 17 in eastern Ukraine, on Thursday, July 24. The passenger plane was shot down July 17 above Ukraine. All 298 people aboard were killed, and much of what they left behind was scattered in a vast field of debris. Hide Caption 1 of 16 Photos: MH17: What they left behind MH17: What they left behind – A classical music record is seen among the sunflowers on July 24. Hide Caption 2 of 16 Photos: MH17: What they left behind MH17: What they left behind – A shoe, appearing to be brand new, sits under foliage at the crash site. Hide Caption 3 of 16 Photos: MH17: What they left behind MH17: What they left behind – Two Dutch passports belonging to passengers lie in a field at the site of the crash on Tuesday, July 22. Hide Caption 4 of 16 Photos: MH17: What they left behind MH17: What they left behind – Clothing, sunglasses and chocolate are seen on July 22. Hide Caption 5 of 16 Photos: MH17: What they left behind MH17: What they left behind – More sunglasses and a travel guide lie in the field on July 22. Hide Caption 6 of 16 Photos: MH17: What they left behind MH17: What they left behind – A doll is seen on the ground on Saturday, July 19. Hide Caption 7 of 16 Photos: MH17: What they left behind MH17: What they left behind – A single shoe is seen among the debris and wreckage on July 19. There has been concern that the site has not been sealed off properly and that vital evidence is being tampered with. Hide Caption 8 of 16 Photos: MH17: What they left behind MH17: What they left behind – Pieces of a wristwatch lie on a plastic cover at the crash site. Hide Caption 9 of 16 Photos: MH17: What they left behind MH17: What they left behind – A toy monkey. Hide Caption 10 of 16 Photos: MH17: What they left behind MH17: What they left behind – Books, bags, a tourist T-shirt. Ukraine's government said it had received reports of looting, and it urged relatives to cancel the victims' credit cards. But a CNN crew at the scene July 19 said it did not see any signs of looting. Hide Caption 11 of 16 Photos: MH17: What they left behind MH17: What they left behind – Passports were scattered across the large field. Hide Caption 12 of 16 Photos: MH17: What they left behind MH17: What they left behind – Playing cards and euros are seen at the crash site. Hide Caption 13 of 16 Photos: MH17: What they left behind MH17: What they left behind – A travel guide and toiletries. Hide Caption 14 of 16 Photos: MH17: What they left behind MH17: What they left behind – Luggage on Friday, July 18. Hide Caption 15 of 16 Photos: MH17: What they left behind MH17: What they left behind – An empty suitcase is cordoned off near the plane's impact site on Thursday, July 17. Hide Caption 16 of 16

JUST WATCHED Ukraine: How did we get there? Replay More Videos ... MUST WATCH Ukraine: How did we get there? 02:30

JUST WATCHED MH17: Tragedy, blame and heartache Replay More Videos ... MUST WATCH MH17: Tragedy, blame and heartache 02:22

JUST WATCHED Ukraine P.M.: Bring 'bastards' to justice Replay More Videos ... MUST WATCH Ukraine P.M.: Bring 'bastards' to justice 02:14

Lack of access to the crash site worries U.S. officials, including Ben Rhodes, deputy national security adviser for strategic communications, who tweeted: "Monitors should be able to access the crash site of MH17. US is deeply concerned by reports that separatists are denying access."

Bociurkiw said the investigation will be difficult because the plane crashed in a difficult-to-access area in the countryside with no electricity.

"I don't think too much of the crime scene has been compromised already," Bociurkiw said. "The bodies are still there. They have not been tampered with. We actually spoke to some civilian emergency workers. They said their job was just to mark where the bodies are."

The FBI is sending two investigators to work on the case, a U.S. law enforcement official said, but the Ukraine government will be in charge of the investigation.

Obama puts focus on Russia

The location of the flight recorder has not been determined.

Ukrainian Economy and Trade Minister Pavlo Sheremeta told CNN's Richard Quest that MH17's black boxes are in Ukraine. Sheremeta would not say, though, whether the government has them.

"They are on Ukrainian territory. I don't know whether we have it or the international team has it," he said.

Earlier Friday, the adviser to the exiled governor of Dontesk told CNN's Victoria Butenko that the black boxes had been retrieved and were in rebel hands, though the location was not clear.

Russia likely bears some of the responsibility for the apparent downing of Flight 17, President Barack Obama said.

In the administration's strongest words yet on the downing of the jet, Obama said rebel fighters couldn't have operated the surface-to-air missile believed responsible for the shootdown "without sophisticated equipment and sophisticated training, and that is coming from Russia."

In his remarks to reporters, Obama said that he did not want to get ahead of the facts of who may have been directly involved in the airliner's destruction. But he said the United States would work hard to hold accountable those responsible for it.

He and other U.S. officials stopped short of publicly placing the responsibility on Russia, which has denied involvement in the destruction of the jetliner.

But a senior defense official told CNN that the "working theory" among U.S. intelligence analysts is that the Russian military supplied the Buk missile system to rebel fighters inside Ukraine.

The United States believes the missile system was transferred into eastern Ukraine from Russia "in recent days or weeks," a senior administration official told CNN, and that the system was operational at the time.

The United States believes pro-Russian separatists could not have operated it without Russian training, the official said, noting that it's unknown whether Russian personnel were on scene when the plane was shot down.

U.S. officials believe the plane was "likely downed by a surface-to-air missile ... operated from a separatist-held location in eastern Ukraine," the U.S. ambassador to the United Nations, Samantha Power, told an emergency session of the U.N. Security Council. If pro-Russian separatists are responsible for shooting down the plane with a missile, investigators can't rule out the possibility that Russia offered help to operate the system, she said.

JUST WATCHED U.S.: Can't rule out Russian involvement Replay More Videos ... MUST WATCH U.S.: Can't rule out Russian involvement 01:51

JUST WATCHED Confusion, hostility at MH17 crash site Replay More Videos ... MUST WATCH Confusion, hostility at MH17 crash site 03:14

JUST WATCHED Video shows the moment MH17 crashed Replay More Videos ... MUST WATCH Video shows the moment MH17 crashed 02:22

Photos: Malaysia Airlines Flight 17 crashes in Ukraine Photos: Malaysia Airlines Flight 17 crashes in Ukraine Debris from Malaysia Airlines Flight 17 sits in a field at the crash site in Hrabove, Ukraine, on September 9, 2014. The Boeing 777 was shot down July 17, 2014, over Ukrainian territory controlled by pro-Russian separatists. All 298 people on board were killed. In an October 2015 report, Dutch investigators found the flight was shot down by a warhead that fit a Buk rocket, referring to Russian technology, Dutch Safety Board Chairman Tjibbe Joustra said. Hide Caption 1 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine Australian and Dutch experts examine the area of the crash on August 3, 2014. Hide Caption 2 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine A woman walks with her bicycle near the crash site on August 2, 2014. Hide Caption 3 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine Police secure a refrigerated train loaded with bodies of passengers from Malaysia Airlines Flight 17 as it arrives in a Kharkiv, Ukraine, factory on July 22, 2014. Hide Caption 4 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine A pro-Russian rebel passes wreckage from the crashed jet near Hrabove on Monday, July 21, 2014. Hide Caption 5 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine – Wreckage from the jet lies in grass near Hrabove on July 21, 2014. Hide Caption 6 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine A man covers his face with a rag as members of the Organization for Security and Co-operation in Europe and the Dutch National Forensic Investigations Team inspect bodies in a refrigerated train near the crash site in eastern Ukraine on July 21, 2014. Hide Caption 7 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine Emergency workers carry a victim's body in a bag at the crash site on July 21, 2014. Hide Caption 8 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine A piece of the plane lies in the grass in eastern Ukraine's Donetsk region on July 21, 2014. Hide Caption 9 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine An armed pro-Russian rebel stands guard next to a refrigerated train loaded with bodies in Torez, Ukraine, on Sunday, July 20, 2014. Hide Caption 10 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine Ukrainian State Emergency Service employees sort through debris on July 20, 2014, as they work to locate the deceased. Hide Caption 11 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine A woman covers her mouth with a piece of fabric July 20, 2014, to ward off smells from railway cars that reportedly contained passengers' bodies. Hide Caption 12 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine Toys and flowers sit on the charred fuselage of the jet as a memorial on July 20, 2014. Hide Caption 13 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine People search a wheat field for remains in the area of the crash site on July 20, 2014. Hide Caption 14 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine A woman walks among charred debris at the crash site on July 20, 2014. Hide Caption 15 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine Emergency workers load the body of a victim onto a truck at the crash site on Saturday, July 19, 2014. Hide Caption 16 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine Emergency workers carry the body of a victim at the crash site on July 19, 2014. Hide Caption 17 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine A large piece of the main cabin is under guard at the crash site on July 19, 2014. Hide Caption 18 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine Victims' bodies are placed by the side of the road on July 19, 2014, as recovery efforts continue at the crash site. International officials lament the lack of a secured perimeter. Hide Caption 19 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine A man looks through the debris at the crash site on July 19, 2014. Hide Caption 20 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine An envelope bearing the Malaysia Airlines logo is seen at the crash site on July 19, 2014. Hide Caption 21 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine Armed rebels walk past large pieces of the Boeing 777 on July 19, 2014. Hide Caption 22 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine Ukrainian rescue workers walk through a wheat field with a stretcher as they collect the bodies of victims on July 19, 2014. Hide Caption 23 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine A woman looks at wreckage on July 19, 2014. Hide Caption 24 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine Pro-Russian rebels stand guard as the Organization for Security and Co-operation in Europe delegation arrives at the crash site on Friday, July 18, 2014. Hide Caption 25 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine A woman walks through the debris field on July 18, 2014. Hide Caption 26 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine Pro-Russian rebels stand guard at the crash site. Hide Caption 27 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine Wreckage from Flight 17 lies in a field in Shaktarsk, Ukraine, on July 18, 2014. Hide Caption 28 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine A man covers a body with a plastic sheet near the crash site on July 18, 2014. The passengers and crew hailed from all over the world, including Australia, Indonesia, Malaysia, Germany and Canada. Hide Caption 29 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine A diver searches for the jet's flight data recorders on July 18, 2014. Hide Caption 30 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine Coal miners search the crash site. Hide Caption 31 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine Wreckage from the Boeing 777 lies on the ground July 18, 2014. Hide Caption 32 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine People search for bodies of passengers on July 18, 2014. Hide Caption 33 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine A woman walks past a body covered with a plastic sheet near the crash site July 18, 2014. Hide Caption 34 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine Belongings of passengers lie in the grass on July 18, 2014. Hide Caption 35 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine People inspect the crash site on Thursday, July 17, 2014. Hide Caption 36 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine People walk amid the debris at the site of the crash. Hide Caption 37 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine Debris smoulders in a field near the Russian border. Hide Caption 38 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine Fire engines arrive at the crash site. Hide Caption 39 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine A man stands next to wreckage. Hide Caption 40 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine Debris from the crashed jet lies in a field in Ukraine. Hide Caption 41 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine Family members of those aboard Flight 17 leave Schiphol Airport near Amsterdam, Netherlands. Hide Caption 42 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine A large piece of the plane lies on the ground. Hide Caption 43 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine Luggage from the flight sits in a field at the crash site. Hide Caption 44 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine A couple walks to the location at Schiphol Airport where more information would be given regarding the flight. Hide Caption 45 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine Flight arrivals are listed at the Kuala Lumpur International Airport in Sepang, Malaysia. Hide Caption 46 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine Debris from the Boeing 777, pictured on July 17, 2014. Hide Caption 47 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine A man inspects debris from the plane. Hide Caption 48 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine Wreckage from the plane is seen on July 17, 2014. Hide Caption 49 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine A man talks with security at Schiphol Airport on July 17, 2014. Hide Caption 50 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine Wreckage burns in Ukraine. Hide Caption 51 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine A man stands next to the wreckage of the airliner. Hide Caption 52 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine People inspect a piece of wreckage believed to be from Malaysia Airlines Flight 17. This image was posted to Twitter Hide Caption 53 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine People inspect a piece of wreckage believed to be from Malaysia Airlines Flight 17. This image was posted to Twitter. Hide Caption 54 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine A piece of wreckage believed to be from Malaysia Airlines Flight 17. This image was posted to Twitter Hide Caption 55 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine A piece of wreckage believed to be from MH17. This image was posted to Twitter Hide Caption 56 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine An airsickness bag believed to be from MH17. This image was posted to Twitter Hide Caption 57 of 58 Photos: Malaysia Airlines Flight 17 crashes in Ukraine A piece of wreckage believed to be from MH17. This image was posted to Twitter Hide Caption 58 of 58

Power also said Russia should take steps to cool tensions in Ukraine.

"Russia can end this war," she said. "Russia must end this war.

At least one American on board

Obama confirmed that at least one U.S. citizen was aboard the plane; Quinn Lucas Schansman was a student at International Business School Hogeschool van Amsterdam, according to his Facebook page. A majority of the passengers (at least 173) were Dutch.

"No one can deny the truth that is revealed in the awful images that we all have seen, and the eyes of the world are on eastern Ukraine, and we are going to make sure that the truth is out," Obama said.

He called for an immediate cease-fire in the region and for a "credible international investigation" into what happened.

Among the evidence cited by U.S. officials and others for their conclusions was an audio recording released by Ukrainian intelligence officials which purportedly feature pro-Russian rebels and Russian military officers discussing a surface-to-air strike and the crash of a civilian jetliner.

"How are things going there," a man identified as a Russian intelligence agent asks.

"Well, we are 100% sure that it was a civilian plane," a man identified as a pro-Russian fighter responds.

"Are there a lot of people?" the Russian officer asks.

The rebel fighter then utters an obscenity and says, "The debris was falling straight into the yards."

CNN cannot confirm the authenticity of this audio, or other similar recordings.

Also, in a news conference Friday, the chief of Ukraine's security service, Valentyn Nalyvaichenko, said the Buk missile system that shot down the airliner crossed the border from Russia only "right before" the attack. He didn't say how investigators know that, however

Ukraine's Interfax news agency reported claims by an adviser to Ukrainian Minister of Internal Affairs Anton Geraschenko that the launcher, as well as the flight data recorders from MH17, were handed over to Russian agents across the border at a checkpoint in the Luhansk area overnight.

A senior Ukrainian official who spoke to CNN also accused Russia of carrying out a cover-up of its role in the shoot-down.

He cited video showing a Buk launcher being moved toward Russia overnight.

CNN could not independently confirm the claims.

Russia-Ukraine dispute

Tensions have been high between Ukraine and Russia since street protests forced former pro-Moscow President Viktor Yanukovych from power in February. Russia subsequently annexed Ukraine's southeastern Crimea region, and a pro-Russian separatist rebellion has been raging in Ukraine's eastern Luhansk and Donetsk regions.

Ukraine's government has accused Russia of allowing weapons and military equipment, including tanks, to cross the border illegally into the hands of pro-Russian rebels.

JUST WATCHED First images of Malaysia Airlines debris Replay More Videos ... MUST WATCH First images of Malaysia Airlines debris 03:11

Map: Approximate route of MH17

Wreckage thought to be from Malaysia Airlines Flight 17 lies in Ukraine on Thursday. This image was posted to Twitter.

While Ukrainian officials implicated pro-Russian fighters and their Russian backers for the jetliner's downing, Moscow argued Ukraine was to blame.

"With regard to the claims raised by Kiev, that it was almost us who did it: In fact I haven't heard any truthful statements from Kiev over the past few months," Russian Foreign Minister Sergey Lavrov said in an exclusive interview with the state-run Russia 24 TV channel.

European Union leaders agreed this week to expand sanctions against individuals and entities in response to Russia's actions in Ukraine, with details to be decided by the end of the month. Expanded U.S. sanctions were also announced in Washington.

Airspace closed

The Ukrainian Ministry of Infrastructure announced Friday that the airspace over Donetsk, Luhansk and part of Kharkiv where rebels are operating had been closed indefinitely.

Malaysian Transport Minister Liow Tiong Lai defended the routing of the Malaysia Airlines plane over the region, saying other carriers were sending their aircraft through the same airspace.

Three months ago, the U.S. Federal Aviation Administration prohibited U.S. airlines from flying in areas some way south of where Flight 17 crashed Thursday. Thursday night, the FAA expanded the flight restrictions to all of eastern Ukraine.

Airline's troubles

Thursday's crash marks the second time this year that Malaysia Airlines has faced an incident involving a downed plane.

In March, Malaysia Airlines Flight 370 disappeared with 239 people on board. Searchers have found no trace of the Boeing 777 or its passengers despite extensive search efforts.

Flight 370 probably flew into the southern Indian Ocean on autopilot with an unresponsive crew, Australian authorities said last month. A new underwater search is expected to begin in August.