Three Dutch investigators have examined bodies from the crashed Malaysia Airlines passenger plane, which are being kept on a train in east Ukraine.
The experts said the train may later leave the rebel-held town of Torez to start identification process.
The US and other nations say there is growing evidence of Russian complicity in the downing of the plane last week. All 298 people on board MH17 died.
Meanwhile, heavy fighting is reported in the main rebel-held city of Donetsk.
Reports say clashes – involving heavy weapons – are going near the city’s airport and the railway station.
At least three civilians were reported killed and one multi-storey building was seen on fire.
BBC correspondents on the ground spoke of a number of refugees fleeing the city.
The Dutch experts are the first international investigators to arrive in the region where the Boeing 777 went down on July 17.
Monitors from the Organisation for Security and Co-operation in Europe have been at the accident site, but their access to the wreckage has been limited by the rebels.
On Monday, the Dutch experts examined some of the 196 bodies kept in refrigerator wagons in Torez, some 15km (9 miles) away from the crash site.
A second train arrived there on Sunday to take more bodies on board.
Pressure has been steadily growing on pro-Russian rebels to allow experts access to the site.
Flight MH17 crashed 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 on June 17. It denies the allegations.
Dutch Prime Minister Mark Rutte, whose country lost 193 people, said all political and economic options were on the table if access to the crash site remained unsatisfactory.
“We want our people back,” he told parliament in The Hague.
Australia’s Foreign Minister Julie Bishop earlier 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.
Separately, US Secretary of State John 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 – also known as BUK – had been transferred to the rebels, 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,” Kerry said on a US TV network.
He threatened further sanctions on Russia and called on European allies to get tougher with President Putin after the “wake-up call”.
Meanwhile, UK Prime Minister David Cameron said Europe and the West “must fundamentally change our approach to Russia” if Putin “does not change his approach to Ukraine”.
The rebels say they will hand over MH17′s flight recorders to the International Civil Aviation Organisation, but the US state department said rebels had tampered with other potential evidence.
Heavy machinery could be seen moving plane debris around at the crash site on Sunday.