DONETSK, Ukraine - Pro-Russia rebels removed some of the bodies of passengers aboard the downed Malaysia Airlines jet from the crash site to be "humane," the separatists' leader told NBC News.

Alexander Borodai, prime minister of the self-declared People's Republic of Donetsk, said that passengers had been "lying out in the heat." However, his forces kept international monitors away from wreckage for a fourth day on Sunday.

The rebels also forced emergency workers at gunpoint to hand over 196 bodies recovered from the crash site and then had them loaded onto refrigerated train cars, Ukrainian officials and monitors told The Associated Press on Sunday.

Alexander Pilyushny, an emergency worker combing the crash site for body parts, said the he and other workers had no choice but to hand the bodies over to the rebels. "They are armed and we are not," Pilyushny said. "The militiamen came, put the bodies onto the trucks and took them away." It was not clear where the train could go.

Employees and local residents say this train in Torez, Ukraine, contains the bodies of passengers of the crashed Malaysia Airlines Boeing 777. MAXIM ZMEYEV / Reuters

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But Borodai insisted his forces were acting compassionately.

"We have already refused to wait for the [international] experts and had to start clearing the bodies from the scene of the event, because waiting longer was contradictory to what it means to be humane," Borodai added. "We are taking all the bodies to refrigerators at the moment which are nearby to the place of tragedy. We hope that finally the necessary experts will arrive there and join our experts and will start the work of identifying the bodies."

Borodai, who is among the rebel leaders who had travel bans and asset freezes imposed by the European Union earlier this month, added that the militants who control access to the scene were "ready to hand over the bodies of the victims to Russia, since it is no more than 10 kilometers [six miles] to the border with Russia, and ... because we believe Russia and believe the Russian authorities."

Alexander Borodai, the self-proclaimed prime minister of the "Donetsk People's Republic," stands next to a pro-Russian separatist wearing military fatigues on Saturday. BULENT KILIC / AFP - Getty Images

But he insisted that pro-Moscow forces would not keep the humans remains from being returned to Western officials, adding "we are ready to hand over the bodies to those institutions that are on the side of our enemy."

When asked by NBC News whether he was in contact with Russian authorities, Borodai replied: "Officially, no." Unofficially? "No comment," he said.

Borodai also told reporters that his forces had not found the Boeing 777's black box flight recorders. He maintains that his forces didn't have the military capability to shoot as high as the Malaysia Airlines jet was flying and blames the Ukrainian military for downing it.

Russian President Vladimir Putin has urged the rebels to cooperate and insisted that a U.N.-mandated investigation must not leap to conclusions. Moscow denies involvement and has pointed a finger at Kiev's military.

Driving home its assertion that the jet carrying 298 people was hit by a Russian SA-11 radar-guided missile, Ukraine's Western-backed government said it had "compelling evidence" the battery was not just brought in from Russia but manned by three Russian citizens who had now taken the truck-mounted system back over the border.

Debris remains littered at the scene of the Malaysia Airlines MH17 crash in eastern Ukraine on Sunday. Albina Kovalyova / NBC News

"The terrorists, with the help of Russia, are trying to destroy evidence of international crimes," the Ukrainian government said in a statement. "The terrorists have taken 38 bodies to the morgue in Donetsk," it said, accusing people with "strong Russian accents" of threatening to conduct autopsies.

Secretary of State John Kerry told his Russian counterpart that the United States was "very concerned" over reports that the remains of victims and debris had been removed or tampered with, the State Department said. He said Washington was also concerned over denial of "proper access" for international investigators and Western observers.

.@JohnKerry reacts to reports of rebels removing bodies and evidence from #MH17 crash site: “What's happening is really grotesque.” #MTP -David Gregory (@davidgregory) July 20, 2014

"This is unacceptable and an affront to all those who lost loved ones and to the dignity the victims deserve," Kerry's spokeswoman, Jen Psaki, said in a statement. "We urge Russia to honor its commitments and to publicly call on the separatists to do the same."

Reuters and The Associated Press contributed to this report.