Ukraine has launched a heavy assault on the rebel-held city of Donetsk, as international investigators arrived in the region to examine the remains of 298 people killed in the downing of a Malaysia Airlines airliner.

A separatist leader on Monday said Ukrainian government forces were trying to break into Donetsk and fighting was under way near the city's railway station. It is the heaviest outbreak of violence in the region since the MH17 airliner was shot down on Thursday.

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Sergei Kavtaradze, of the self-proclaimed Donetsk People's Republic, said at least four tanks and armoured vehicles were attacking his men. Two rebel tanks were reported heading to Donetsk railway station. 

A Ukrainian military spokesman said the operation was in progress but he would not comment on troop movements.

The city of Donetsk is about 80km from the crash site of MH17. Allies of Kiev have said that the security situation in the area was hampering the investigation into the downing of the aircraft.

Three members of a Dutch victim identification team arrived in the region on Monday, and were expected to visit a railway station near the crash site where nearly 200 bodies have been stored in refrigerated wagons.

Rescuers said they had found a total of 251 bodies and 86 body fragments at the crash site and said a second refrigerated train had arrived.

Ukraine's prime minister, Arseniy Yatsenyuk, said Ukraine was prepared to hand over all evidence to the Dutch, and said the Netherlands should lead the inquiry into the airliner's destruction.

However, rebels control the crash site, and the whereabouts of the plane's flight recorders is unknown. 

On Sunday the US secretary of state, John Kerry, said he had "overwhelming evidence" that Russia had supplied and trained separatists on a surface-to-air missile system that the US says was used to shoot down the airliner.

"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 the CBS news network.

The evidence has not been made public.

Source: Al Jazeera and agencies