Ukrainian army tanks were reported to be launching an assault to break pro-Russian rebels' hold on the eastern city of Donetsk on Monday in the first major outbreak of hostilities in the area since Malaysia Airlines flight MH17 was shot down last week.
A separatist leader said Ukrainian government forces were trying to break into Donetsk and fighting was under way near the railway station.
Sergei Kavtaradze, of the rebels' self-proclaimed Donetsk People's Republic, said at least four tanks and armored vehicles were trying to break through into the city.
A Ukrainian military spokesman said 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," Vladyslav Seleznyov said.
Journalists also reported seeing two rebel tanks heading towards Donetsk railway station.
As international horror deepened over the fate of the remains of the 298 victims of the air disaster, the first international investigators reached rebel-held territory in eastern Ukraine on Monday.
Three members of a Dutch disaster victims identification team arrived in Donetsk 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 a second refrigerated train had arrived.
The shooting down of the airliner on Thursday has sharply deepened the Ukrainian crisis, in which separatists in the Russian-speaking east have been fighting government forces since protesters in Kiev forced out a pro-Moscow president and Russia annexed Crimea in March.
The U.S. and its allies have pointed the finger at the pro-Russian rebels and at Moscow itself over the downing of the plane although Russia has denied involvement.