The United States has confirmed that Russia supplied sophisticated missile launchers to separatists in eastern Ukraine and that attempts were made to move them back across the Russian border after the Thursday shoot-down of a Malaysian jet liner, a U.S. official said Saturday.

“We do believe they were trying to move back into Russia at least three Buk [missile launch] systems,” the official said. U.S. intelligence was “starting to get indications . . . a little more than a week ago” that the Russian launchers had been moved into Ukraine, said the official.

The official’s comments, made on condition of anonymity to speak about intelligence matters, came as a top Ukrainian counterintelligence official said his service has conclusive proof that Russia supplied the missile that shot down Malaysia Airlines Flight 17 over territory controlled by the separatists.

Aviation investigators from around the world were converging on Kiev on Saturday hoping to begin their work, but it remained unclear when they would gain full access to a mammoth site deep in rebel-held territory in the eastern part of the country. Ukrainian officials warned that the chance for an impartial inquiry was quickly slipping away as bodies were moved and at least some plane remnants were loaded onto trucks.

International observers were allowed only brief access to the site on Saturday and were restricted in their movements by the heavily armed rebels, some of whom appeared drunk, witnesses said.

Michael Bociurkiw, spokesperson of Organization for Security and Cooperation in Europe, describes to the journalists the scene at the crash site of Malaysia Airlines Flight MH17. He said that the location had a heavy security presence and that “they were being watched very carefully.” (Divya Jeswani Verma/The Washington Post)

“Their key task is to destroy possible evidence,” said Andriy Parubiy, head of Ukraine’s National Security and Defense Council. “It will be hard to conduct a full investigation with some of the objects being taken away, but we will do our best.”

Ukraine and Western officials have said that Russia is providing support and equipment to the rebels.

The Kremlin has denied that it has sent weapons to the rebels, and it has continued to take a strong line against the West even after the plane crash, issuing sanctions Saturday against 13 Americans in retaliation for U.S. sanctions that were announced the day before Thursday’s attack on the plane.

Ukrainian officials said that at least 38 of the 192 bodies that have been discovered had been removed from the scene and taken to the nearby rebel-held city of Donetsk.

Temperatures have been in the 80s and the bodies have been rapidly decomposing, witnesses said.

Konstantin Batozsky, an adviser to Serhiy Taruta, governor of the Donetsk region, said these actions by the rebels were meant to undermine an independent investigation and to “make all the procedures illegitimate.”

Vitaly Nayda, counterintelligence chief of Ukraine’s security service, offered photographs and said Ukraine has evidence of the movement of three Buk M-1 antiaircraft missile systems from rebel-held territory into Russian territory early Friday, less than

12 hours after the plane was downed. Ukrainian officials have said that a missile from a Buk M-1 launcher was used to shoot down the aircraft.

View Graphic Aviation history is littered with civilian planes that were shot from the sky, intentionally or not, by military weapons. Malaysia Flight 17 was cruising at 33,000 feet, more than half a mile higher than Mt. Everest, when a missile hit it July 17. And the missile’s range is believed to be more than twice that high.

Two of the antiaircraft systems were spotted entering Russia from Ukraine at 2 a.m. Friday, he said. One had its full complement of four missiles, but the other was missing a missile, he said. Two hours later, he said, a convoy of three vehicles that included one of the launchers and a control truck crossed into Russia.

The U.S. official said they could not confirm the exact time cited by the Ukrainians.

Nayda said that Ukrainian military services had not left any operational Buk M-1 launchers in territory where the rebels could have seized them when they took over bases and territory in eastern Ukraine this year. He suggested they must have come from Russia and said Ukraine has evidence that at least one launcher system was on its territory Monday.

The rebels have denied possessing the launchers, although social media files linked to a rebel leader, Igor Girkin, appeared to boast of having the systems. The claims were deleted this week after the plane was shot down.

A top rebel leader said Saturday that his side was not tampering with the evidence, even as rebels on the scene appeared to be loading at least some parts of the plane onto trucks. The leader said he was eager for international investigators to come as soon as possible.

“Currently in this area there are no active hostilities,” Alexander Borodai told reporters in Donetsk. “But the situation may deteriorate at any time.”

Fighting raged elsewhere in the region Saturday, especially in Luhansk near the Russian border, where 16 civilians were killed, according to the city council’s Web site.

The attack on the plane and the subsequent treatment of the crime scene appear to be hardening European attitudes against Russia.

Most of the 298 victims were Dutch citizens, and the chaos Saturday drew a harsh condemnation from Dutch Prime Minister Mark Rutte, who said he had told Russian President Vladmir Putin “that the opportunity is fading to quickly show the world that he is serious about wanting to help.” The Netherlands had previously been cautious about criticizing Russia, a major trading partner.

Rutte also lashed out at the rebels, saying he was “shocked by the images of completely disrespectful behavior” at the crash site. “This is outright disgusting,” he said.

German Chancellor Angela Merkel spoke to Putin on Saturday, asking him to “use his influence on the separatists” to arrange a cease-fire to allow investigators to pursue their work, a step the Kremlin said it supported.

Liow Tiong Lai, Malaysia’s minister of transportation, said his government is “deeply concerned the crash site not been properly secured and the integrity of the site has been compromised.” Blocking access to the site “cannot be tolerated,” he said.

Russia and the rebels have denied any involvement in the attack on the Boeing 777, which killed 192 Dutch citizens, 44 Malaysians, 27 Australians, 12 Indonesians, 10 Britons, four Germans, four Belgians, three Filipinos, one Canadian and one person from New Zealand. One passenger held dual Dutch-U.S. citizenship.

In an op-ed column in the Sunday Times, British Prime Minister David Cameron said: “The growing weight of evidence points to a clear conclusion: that flight MH17 was blown out of the sky by a surface-to-air missile fired from a rebel-held area. If this is the case then we must be clear what it means: this is a direct result of Russia destabilising a sovereign state, violating its territorial integrity, backing thuggish militias and training and arming them.”

Cameron then called on Europe’s leaders to take action, saying that “for too long there has been a reluctance on the part of too many European countries to face up to the implications of what is happening in eastern Ukraine.”

The pro-Russian separatists had said Friday that they would allow the victims’ bodies to be transported out of rebel-held territory because they did not have enough refrigerated facilities for all of them. But Ukrainian officials said Saturday that they were still trying to negotiate safe passage for teams of investigators and international observers.

A spokesman for the Organization for Security and Cooperation in Europe said a team of 24 international observers had seen people moving bodies and putting them in body bags. The team was sharply restricted in what it could do and see, he said.

Rebels “have what they describe as experts, so-called experts here,” OSCE spokesman Michael Bociurkiw said. “They’ve brought body bags and they’re moving the bodies to the side of the road, as far as we can tell.”

“We don’t know who they are,” Bociurkiw said of the people moving the bodies. “We are unarmed civilians, so we’re not in a position to argue heavily with people with heavy arms.”

Karen DeYoung in Washington; Ferry Biedermann in Amsterdam; Annie Gowen in Kuala Lumpur, Malaysia; and Karoun Demirjian in Moscow contributed to this report.