mh17 bodies
Monitors stand near a refrigerator wagon containing bodies of MH17 victims at a railway station in Torez Getty

Fighting has erupted in the rebel-stronghold city of Donetsk as the Ukrainian army attempts to break through, pro-Russian separatists have reported.

Ukrainian forces launched an assault on the rebels near the railway station in the city on Monday, with witnesses reporting loud explosions and plumes of smoke rising from the area.

A separatist leader said Ukrainian forces were trying to break into Donetsk, to take back the city seized by rebels in April.

Clashes have been reported on the Donetsk city limits, where the remains of some of the victims of Malaysia Airlines MH17 are believed to be held after their bodies were collected from the crash site on the outskirts of the region.

According to Reuters, a witness saw two rebel tanks heading towards the railway station, driving against a steady flow of people running away.

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 he has called an "anti-terrorist operation" against the rebels.

An official from the self-titled Donetsk People's Republic, Sergei Kavtaradze, said that approximately four tanks were trying to gain access into the city.

"Attention! There is fighting in the Krasnoarmeiskoye highway, roads Marshal Zhukov, Stratonavtov-Artyomovskaya streets. In the square ... there is also fighting," a city council statement said.

"We ask residents living in this square not to go out and not to leave their homes and apartments."

A Ukrainian military spokesman told The Independent that its operations in the east of the country were ongoing, but he could not confirm the reports of soldiers entering Donetsk.

"The active phase of the anti-terrorist operation is continuing. We are not about to announce any troop movements," Vladyslav Seleznyov said.

On Sunday, up to 200 bodies were loaded by pro-Russian separatists onto a train with refrigerated compartments in the town of Torez, which was headed for Donetsk.

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

The separatists and Ukraine are accusing each other of firing a surface-to-air missile at the passenger jet MH17 as it flew from Amsterdam to Kuala Lumpur on Thursday, although both sides deny responsibility for the downed plane.