Ukraine's Deputy Defence Minister says its forces are "shifting to offensive actions" in some areas.
Ukrainian military sources told the BBC that a series of small scale armoured offensive operations are underway.
They did not say whether this was the start of Ukraine's long awaited full scale counter-offensive.
Earlier, Russia's defence ministry said it thwarted a major Ukrainian attack in Donetsk.
BBC Ukraine correspondent James Waterhouse spoke to Corin Dann.
