Fighting raged near the railway station in the rebel-held city of Donetsk in eastern Ukraine on Monday, and witnesses saw plumes of smoke and heard loud explosions from the area around the station in the city centre.
A pro-Russian separatist leader said Ukrainian forces were trying to break into Donetsk and a Reuters 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 Kiev calls its "anti-terrorist operation" against the separatists.
"Attention! There is fighting in the Krasnoarmeiskoye highway, roads Marshal Zhukov, Stratonavtov-Artyomovskaya (streets). In the square ... there is also fighting," the Donetsk city council said in a statement.
"We ask residents living in this square not to go out and not to leave their homes and apartments."
Sergei Kavtaradze, an official of the rebels' self-proclaimed Donetsk People's Republic, said there were at least four tanks and armoured vehicles trying to break through into the city.
A spokesman for Ukraine's military operations in eastern Ukraine said the operation was in an "active phase" but could not comment on reports of troops entering Donetsk because he did not want to give away the Ukrainian strategy.
A Malaysian airliner was shot down in eastern Ukraine on Thursday, increasing calls for an end to fighting in the Russian-speaking region.