Ukraine launched a military assault on Monday to break pro-Russian rebels' hold on the eastern city of Donetsk in the first major hostilities in the area since Malaysia Airlines Flight MH17 was shot down last week.

As Dutch investigators arrived to inspect the bodies of hundreds of victims near the crash site, the fighting in Donetsk served as a reminder of the dangers they face working in a war zone.

Artillery fire sent plumes of smoke skyward near the Donetsk railway station, about 40 miles from the crash site, in what the separatists said was an attempt by government forces to enter the city, which they seized in April.

Sergei Kavtaradze of the rebels' self-proclaimed Donetsk People's Republic said at least four army tanks and armored vehicles were trying to break through into the city.

A Ukrainian military spokesman confirmed that the operation was in progress but would not comment on reports of troops entering Donetsk. "The active phase of the anti-terrorist operation is continuing. We are not about to announce any troop movements," said Vladyslav Seleznyov.

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.

Against a background of international horror over the fate of the remains of the 298 victims of the Malaysia Airlines disaster, the first international investigators reached eastern Ukraine on Monday.

Three members of a Dutch disaster victim identification team arrived at a railway station near the crash site where rebels say 247 bodies have been stored in refrigerated wagons. More than half of the crash victims were Dutch.

The head of the team inspected the storage of the bodies in the rail cars and, despite an overwhelming stench of decomposition when the doors were opened, said it was fine.

"The storage of the bodies is of good quality," said Peter van Vliet, whose team went through the wagons dressed in surgical masks and rubber gloves.

He said he had been told the train would be leaving the station at Torez later on Monday so the bodies can be taken where they can be identified and eventually repatriated. He could not say where it was going.

Ukrainian officials said that as of Monday morning, 272 bodies and 66 fragments of bodies had been found.

Michael Bociurkiw, a spokesman for the Organization for Security and Cooperation in Europe, said reports from the group's investigators in Ukraine suggest some bodies were incinerated without a trace.

"We're looking at the field where the engines have come down. This was the area which was exposed to the most intense heat. We do not see any bodies here. It appears that some have been vaporized," he said from the crash site.

The Netherlands and Russia agreed Monday that the International Civil Aviation Organization should lead the investigation.

But Dutch Prime Minister Mark Rutte also said Monday that Russian President Vladimir Putin needed to do more to convince the separatists to lay down their arms and allow investigators unfettered access to the crash site.

“It is clear that Russia must use her influence on the separatists to improve the situation on the ground,” he told the Dutch parliament. “If in the coming days access to the disaster area remains inadequate, then all political, economic and financial options are on the table against those who are directly or indirectly responsible for that.”