Story highlights Close to 1,400 people have been killed, and more than 4,000 wounded, since mid-April

"Today, with all certainty, there's a need to speak about a true war," says Russian official

A U.S. military team in Kiev is aiding the recovery effort of downed Flight MH17

NATO official says there are about 20,000 Russian troops near the Ukraine border

A new buildup of Russian troops along the Ukraine border raised concerns Tuesday that Moscow might be contemplating another intervention like the one that annexed Crimea earlier this year.

According to a NATO official, Russia now has about 20,000 troops stationed "in an area along the entire border with eastern Ukraine." The buildup nearly doubled the troop deployment in the last week by adding 8,000 more forces to 12,000 already there, the official said.

It comes a week after the United States and the European Union increased economic sanctions on Russia for supporting pro-Russian separatists fighting Ukraine government forces in the eastern regions of Donetsk and Luhansk, along the border with Russia.

In addition, Russia's Defense Ministry is staging a week of military exercises involving air troops and anti-missile defense forces. The exercises are taking place in Russia's southern Astrakhan region, roughly 500 miles from the border with Ukraine.

Similar military exercises in the region preceded Russia's annexation of Crimea in March, which escalated the Ukraine conflict following the ouster of pro-Moscow President Viktor Yanukovych a month earlier.

Donetsk braces for offensive

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

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Meanwhile, Ukraine government security officials said Monday they were preparing for a "massive assault" on Donetsk city, state media reported . Inside the city, a rebel stronghold for months, shelling has already pushed some residents underground into cellars and half-built basements.

Russia's Foreign Ministry claimed in a statement Monday that the Ukrainian military was firing missiles and using multiple rocket systems in and around the city.

It accused Ukraine's government of wanting to continue the war and called for talks to find a political situation to the crisis.

With escalated fighting and Ukrainian forces making gains, the Russian deployment at the border could portend an intervention under the banner of a peacekeeping operation.

"On a human scale, the situation in the east -- particularly in Donetsk and Luhansk -- is disastrous. Today, with all certainty, there's a need to speak about a true war," Vitaly Churkin, Russia's ambassador to the United Nations, said Tuesday.

Russian troop buildup

The NATO official said Russian forces gathering at the Ukraine border included a "spectrum" of assets: infantry, mechanized divisions, armor, a lot of artillery, conventional and air defense, and special forces and logistics.

"They are very capable Russian regular units and can move in a matter of hours and could significantly disrupt the situation" in eastern Ukraine, the official said.

The Ukrainian government and Western leaders accuse Russia of fomenting instability in its neighbor by arming and supporting the rebels there, which Russia denies.

The downing of Malaysia Airlines Flight 17 on July 17 exacerbated the situation.

Malaysian plane downed

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U.S. and Ukrainian officials allege that a Russian-made missile shot down the plane from rebel-held territory, killing all 298 people on board. Russia and the rebel fighters deny involvement.

Earlier this year, Russia amassed about 40,000 troops on the Ukraine border and threatened to invade, NATO said in a fact sheet published last month

Putin then ordered most of the soldiers to return to their bases, leaving about 12,000 of July 11, NATO said.

On Tuesday, Ukraine's National Defense and Security Council said the Russian deployment now numbered as high as 45,000, and that there had been 26 clashes in the past 24 hours, including six attacks from Russian territory.

Meanwhile, Russian combat aircraft crew will practice firing at air and ground targets in unfamiliar territory, the defense ministry said of the military exercises in the region. The maneuvers will involve 100 aircraft, including fighter jets, bombers and combat helicopters.

In its fact sheet, NATO challenged Russian accusations that the alliance is escalating the crisis in Ukraine by bolstering its support for NATO members in Eastern Europe.

"Over the past months, Russia has also embarked on an unprecedented schedule of no-notice military exercises involving massive numbers of troops and heavy equipment," NATO said, adding: "Russia should explain what its military plans are before it starts accusing others of posing a threat."

Search for remains

An official with knowledge of the MH17 investigation told CNN that a significant part of the crash site was between Ukrainian and separatist front lines, which constantly change.

A 17-vehicle convoy of 110 experts accompanied by nine international monitors made it to the site on Tuesday, the Organization for Security and Co-operation in Europe said via Twitter.

The experts, from the Netherlands, Australia and Malaysia, were hampered in their search efforts Monday by renewed fighting in the area.

Monday's search focused in and around the village of Petropavlivka, in the western part of the huge debris field, but the team was unable to complete its mission after an initial delay.

They did recover some personal belongings of those on the flight, including photo albums and passports, said Pieter-Jaap Aalbersberg, head of the Dutch recovery mission.

"Although some access to the crash site has been secured, we still don't have the unimpeded and complete access to the site -- all of it -- that is essential," Gary Quinlan, Australia's ambassador to the United Nations, said Tuesday.

In addition, a team of about a dozen U.S. military specialists arrived Tuesday in Kiev to assist the recovery effort, Pentagon spokesman Rear Adm. John Kirby told reporters.

"They are military; they will not leave Kiev," he said, adding that the team includes experts in communications, logistics and other areas that can aid the investigation.

"Recovery operations is something, tragically and unfortunately, the U.S. military has to do and has to be good at," Kirby said.

Refugees flee conflict

The ongoing fighting has killed close to 1,400 people -- civilians and combatants -- and more than 4,000 people have been wounded in eastern Ukraine since mid-April, according to U.N. officials.

The fighting has also forced hundreds of thousands of people to flee their homes and seek shelter either elsewhere in Ukraine or across the border in Russia.

The U.N. refugee agency, the UNHCR, said Tuesday that the Russian authorities estimate that around 730,000 Ukrainians have sought sanctuary in Russia this year under a visa-free travel program. This includes more than 168,000 who have reported to Russia's Federal Migration Service.

UNHCR's European director Vincent Cochetel told journalists in Geneva that about 80% of the Ukrainians who have fled to Russia are staying in border areas, while others are with friends or family in other parts of the country.

Many people in eastern Ukraine are Russian-speaking and have strong ties to Russia.

At the same time, nearly 118,000 people are reported to be internally displaced, most of them from eastern Ukraine, based on information given to the UNHCR by Ukrainian authorities and local NGOs.

"In the past seven days more than 6,200 people have been forced from their homes," Cochetel said.

Many have fled with limited possessions, some driven out by the destruction of homes and infrastructure, while others say they fear being caught in the crossfire or suffering persecution.

John Ging, operations director of the U.N. Office for the Coordination of Humanitarian Affairs, called on all sides Tuesday to allow people to move freely and safely.

"The humanitarian situation is deteriorating, and the worrying increase in violence in urban areas puts a greater number of people at risk.

"Until violence is ended, we will continue to see an increase in human suffering, and in the number of people displaced. Immediate action is required to prevent this crisis from worsening," he said, according to prepared remarks.