A pro-Russian separatist leader says Ukrainian military forces are attacking rebel forces in the eastern city of Donetsk.

Sergei Kavtaradze, a self-styled official in the self-proclaimed "Donetsk People's Republic," said at least four Ukrainian tanks and armored vehicles were attempting to fight their way into the city, Reuters reports.

Fighting is also centered around the city's train station, where explosions and plumes of black smoke are reported.

People were seen fleeing the city center, near the train station, where two rebel tanks were reportedly driving toward.

The Donetsk city council issued a statement warning people to remain in their homes and to stay away from several roads and the Krasnoarmeiskoye highway because of fighting.

A Ukrainian military official in Kyiv said the operation is in an "active phase" and he could not comment on it.