The Obama administration expanded its case Sunday accusing Ukrainian separatists and Russian forces of working hand in hand to acquire and operate a missile battery believed to have shot down a Malaysia Airlines jetliner last week, killing nearly 300 people.

Citing an “enormous amount of evidence,” Secretary of State John F. Kerry accused Russia of providing SA-11 antiaircraft missiles to the pro-Russian rebels and training them how to fire the advanced weapons. He also said U.S. intelligence agencies “saw the launch” of a missile from rebel-held territory in eastern Ukraine and recorded its trajectory at the moment the passenger plane vanished from radar.

Meanwhile, in Kiev, the U.S. Embassy said American intelligence analysts had confirmed the authenticity of recorded conversations in which rebel leaders bragged about shooting down what they thought was a Ukrainian military transport plane moments after the Malaysian jetliner was blown apart. The Ukrainian government had previously aired the audio excerpts, but the U.S. statement vouching for their reliability buttressed the charges against the rebels.

The fresh allegations came as armed separatists maintained tight control over the crash site in eastern Ukraine, preventing outside investigators from conducting an independent forensic examination of what caused the midair disaster.

Three days after the crash, Ukrainian rebels moved the decomposing bodies of about 200 victims to refrigerated railway cars at a nearby train station but continued to haggle with their enemies in the Ukrainian government over what to do with the remains.

A flight data recorder is seen being recovered from the crash site of Flight MH17 in eastern Ukraine. (Reuters)

Temperatures in eastern Ukraine have reached the mid-80s, and the bodies were rapidly decomposing in the fields and villages where they fell.

“The stench was absolutely overwhelming” in the refrigerated trains, said Michael Bociurkiw, a spokesman for the Organization for Security and Cooperation in Europe, which has sent international observers to the site. “I don’t want to be too gruesome, but it’s a very difficult scene to watch.”

Bociurkiw said local officials and rebel representatives planned to leave the railway cars in place until international experts could inspect them and provide guidance.

“The thinking is that the cars should be taken to Ukrainian-controlled territory and they should be processed there,” he said.

A spokesman for the rebels, Sergey Kavtaradze, confirmed that the separatists had moved the bodies but said they were ready to hand the remains over to authorities in Kiev. “Finally we’ve placed them in a train, and now we are waiting for the experts to come,” he said. “We’re expecting Kiev to tell us where to take these bodies.”

The rebels had come under withering criticism from officials in Kiev, Washington, Kuala Lumpur, The Hague and European capitals who claimed they had failed to show respect for the dead — most of whom were Dutch citizens, flying from Amsterdam to Malaysia — and failed to preserve the integrity of the crime scene. The separatists have denied shooting down the plane, suggesting that the Ukrainian government was probably responsible.

On Sunday, after days of chaos, the rebels sought to bring a measure of order to the crash site. As many as 800 people were drafted to comb the area for bodies and debris, according to Ukrainian Vice Prime Minister Volodymyr Groysman, who has been leading negotiations with the separatists. The search effort, however, remained under the direct control of the rebels, he said.

View Graphic Before and after the crash of Malaysian Airlines Flight 17

Separatists said they found two devices that they believe are the jetliner’s black boxes, which could contain valuable flight information about the last moments of Malaysia Airlines Flight 17. Kavtaradze, the rebel spokesman, said the devices were under the personal watch of leader Alexander Borodai.

“We have found the technical devices that potentially might be the black box,” Kavtaradze said. “Experts should determine this. We expect Malaysian experts to come today, and we will give the technical devices to them. We will not provide those things to the Ukrainian side.”

The condition of the black boxes — and who ends up controlling them — could play a key role in the crash investigation.

On Sunday, the Ukrainian security service released audio recordings of what they said were rebel leaders talking on the phone two days earlier about the importance of keeping the black boxes out of the hands of international investigators. According to the intercepted calls, whose authenticity could not be independently verified, a rebel commander issued orders at the behest of “our high-placed friends” in Moscow to maintain close watch over the boxes.

U.S. and Ukrainian government officials have accused the rebels of destroying evidence, obstructing outside investigators and covering their tracks. The finger-pointing continued Sunday as Kerry appeared on several network talk shows and blamed the downing of the jet on the separatists and, indirectly, on the Russian government.

On CNN’s “State of the Union,” Kerry asserted that Russian forces had trained the separatists how to use the SA-11 antiaircraft missiles and that over the past month “a major flow of arms and weapons” had crossed the border from Russia into eastern Ukraine.

“We know for certain that the separatists have a proficiency that they’ve gained by training from Russians as to how to use these sophisticated SA-11 systems,” he said.

Kerry said imagery collected by U.S. intelligence agencies showed that a missile was launched Thursday from eastern Ukraine and that it was on a trajectory to strike the Malaysian jetliner. He also cited videos posted on the Internet that purportedly show an SA-11 system passing through two towns in eastern Ukraine “right in the vicinity of the shoot-down” within hours of the crash.

Kerry’s comments came a day after a U.S. official said Washington had confirmed that Russia supplied high-altitude missile launchers to separatists in eastern Ukraine this month and that attempts were made to move them back across the Russian border in the aftermath of Thursday’s aviation disaster.

Ukrainian government officials have asserted that they have proof that Russia provided rebels with the actual missile used to shoot down the airliner. The Obama administration has stopped short of making that claim but has built a steadily expanding circumstantial case that the Russians were at least indirectly involved.

“There’s an enormous amount of evidence, even more evidence than I just documented, that points to the involvement of Russia in providing these systems, training the people on them,” Kerry said on CBS’s “Face the Nation.”

At the same time, Kerry indicated that the United States lacked proof that Moscow was directly responsible for the deaths of the airline passengers. “There’s a stacking up of evidence here that Russia needs to help account for,” he said on NBC’s “Meet the Press.” “We are not drawing the final conclusion here, but there is a lot that points at the need for Russia to be responsible.”

Russian officials have not responded to the accusations that their forces supplied SA-11 units and training to the rebels. In the immediate aftermath of Thursday’s crash, Russian President Vladimir Putin indirectly blamed the government of Ukraine, saying that it “bears responsibility” because it has not ceased hostilities with the separatists.

In a statement on Monday, Putin said international experts should have safe access to the site, yet also seemed to push back against the global accusations implicating rebels.

“No one should and has the right to use this tragedy to achieve their selfish political goals.”

Birnbaum reported from Kiev. Philip Bump in Washington and Karoun Demirjian in Moscow contributed to this report.