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.

Russian President Vladimir Putin said the escalation of the conflict had put Ukraine “on the verge of civil war”.

The Kremlin said Mr Putin made the warning in a phone call with German Chancellor Angela Merkel.

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Analysis

Jonathan MarcusBBC defence and diplomatic correspondent

The most dangerous moment in the Ukrainian crisis so far may well have arrived. The security operations undertaken by the Kiev government have been received by Russia with alarm. “Events are beginning to develop under the worst case scenario,” said one senior official in Moscow. As yet, amidst a torrent of social media and conflicting local reports, it is impossible to determine the extent of any casualties. But in a sense, it is not the facts but the perception of what is happening that matters.

Russia’s narrative has been clear. It sees itself as the champion of Ukraine’s Russian-speakers in the east. With separatists taking over buildings in several Ukrainian cities, the Kiev authorities have effectively been goaded into a response. The question now is whether that response will be seized upon by Moscow as a pretext for military action.

It said that both leaders “emphasised the importance” of planned talks in Geneva on Thursday between Russia, Ukraine, the US and the EU.

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, its industrial heartland.

Thousands of Russian troops are reported to be deployed along the border, kindling fears that any crackdown on the rebels could trigger an invasion.