US Secretary of State John Kerry says there is overwhelming evidence of Russian complicity in the downing of a Malaysia Airlines plane in Ukraine.
Mr Kerry called on Russia to take responsibility for the actions of the rebels, saying their handling of the dead had been "grotesque".
All 298 people on flight MH17 died when it was reportedly hit by a missile.
Russia has been accused of providing the rebels with an anti-aircraft system that was allegedly used in the attack.
Earlier, the remains of up to 196 people were loaded on to refrigerated rail wagons in eastern Ukraine, to be taken to an unknown destination.
Australia's Foreign Minister Julie Bishop has called on pro-Russian separatists not to use the bodies as pawns in their conflict with the Ukrainian authorities.
"There are 298 bodies on that site - their families, their loved ones want them home now," she said.
'Wake-up call'
Mr Kerry said the US had seen major military supplies moving into Ukraine from Russia in the last month, including a convoy of armoured personnel carriers, tanks and rocket launchers.
Intercepted calls suggested a Russian SA-11 missile system had been transferred to the rebels, Mr Kerry said, and the US had seen a video of a launcher being moved back into Russia after flight MH17 crashed.
"There's [an] enormous amount of evidence that points to the involvement of Russia in providing these systems, training the people on them," Mr Kerry said on a US TV network.
He also threatened further sanctions on Russia and called on European allies to get tougher with President Putin after the "wake-up call".
Writing in a newspaper on Sunday, UK Prime Minister David Cameron said Europe and the West "must fundamentally change our approach to Russia" if Mr Putin "does not change his approach to Ukraine".
Source: BBC