"Now's the time for President Putin and Russia to pivot away from the strategy that they've been taking and get serious about trying to resolve hostilities within Ukraine," he said at the White House.

Putin and Russia have a direct responsibility to compel separatists to cooperate with the investigation, and the burden is now on Moscow to insist that separatists stop tampering with the investigation, he said.

European security monitors said gunmen stopped them inspecting the site when they arrived on Friday, and Ukrainian officials said separatists had tampered with vital evidence, allegations echoed by Obama.

"What are they trying to hide?" the U.S. president said.

But the spokesman for the European security monitors said they had had unfettered access on Monday, and three members of a Dutch disaster victims identification team arrived at a railway station near the crash site and inspected the storage of the bodies in refrigerated rail cars.

Peter van Vliet, whose team went through the wagons dressed in surgical masks and rubber gloves, said he was impressed by the work the recovery crews had done, given the heat and the scale of the crash site. "I think they did a hell of a job in a hell of a place," he said.

As they went about their work, fighting flared in Donetsk, some 60 kilometers (40 miles) from the site, in a reminder of the dangers the experts face operating in a war zone.

The government in Kiev denied sending the regular army into the center of Donetsk, which pro-Russian separatists captured in April, but said small "self-organized" pro-Ukrainian groups were fighting the rebels in the city. Four people were killed in clashes, health officials said.

Donetsk is at the heart of a rebel uprising against rule by Kiev, and Ukrainian President Petro Poroshenko has vowed to retake the city as part of what Kiev calls its "anti-terrorist operation" against the separatists. (rt/ez)