Ukraine's acting President Olexander Turchynov has announced the start of an "anti-terrorist operation" against pro-Russian separatists.
He told parliament it was being conducted "stage by stage, in a responsible... manner".
Hours later, gunfire was heard at an airbase which officials said had been in the hands of militants.
Mr Turchynov said the airbase at Kramatorsk had been "liberated" from "terrorists".
Pro-Russian rebels have seized buildings in about 10 towns and cities across Ukraine's eastern provinces, which form the heartland of Ukraine's heavy industry.
Thousands of Russian troops are reported to be deployed along the border, kindling fears that any crackdown on the rebels could trigger an invasion.Russia annexed the Ukrainian province of Crimea last month, after it broke away and held a controversial referendum on self-determination.
The BBC's Gabriel Gatehouse in Kramatorsk says that an angry crowd had gathered outside the airbase, prompted by the arrival of two helicopters carrying soldiers they said had come from Kiev.
A crowd of some 200 people remained on Tuesday evening, chanting slogans in favour of a referendum on the region's future, our correspondent adds.
A spokesman for the Russian foreign ministry expressed "deep concern" at reports of casualties in eastern Ukraine, but these could not be confirmed.
If Ukrainian forces attempt to move on to the town of Sloviansk, they will find what looked like very experienced soldiers on the pro-Russian side, the BBC's Daniel Sandford reports.