Ukrainian armed forces have launched a "special operation" against pro-Russian separatists in the east of the country and recaptured a military airfield.
Gunfire could be heard from the airfield at the town of Kramatorsk on Tuesday after a fighter jet swooped low over the area and Ukrainian troops were seen disembarking from helicopters.
A Reuters correspondent in Kramatorsk saw four military helicopters over the airport. Two of these landed and when troops stepped out and walked across the field, locals manning a barricade shouted at them to go home.
Ukraine's acting President Oleksander Turchinov had earlier announced that a military operation was under way to flush pro-Russian separatists out of the government buildings and facilities they have seized in about 10 towns and cities in the east over the last few days.
Mr Turchinov issued a statement saying Ukraine had retaken the airfield in Kramatorsk from pro-Russian militants, while the state security service said an "anti-terrorist" operation was in progress against separatists in the nearby town of Slaviansk.
The operations appeared to mark an escalation of the deepest East-West crisis since the Cold War.
The standoff has raised fears in the West and in Kiev that Russia might intervene militarily on behalf of Russian speakers in eastern Ukraine, following its annexation of the Crimean region last month in response to the overthrow of Ukraine's pro-Moscow president, Viktor Yanukovich, after weeks of protests.
Moscow on Tuesday declared Ukraine on the brink of civil war as Kiev announced the operation against pro-Russia separatists.
Russia accuses Ukraine of provoking the crisis by ignoring the rights of citizens who use Russian as their first language, and has promised to protect them from attack. Russia also stresses the presence of far-right nationalists among Kiev's new rulers.
However, a United Nations report on Tuesday cast doubt on whether Russian-speakers were seriously threatened.
The UN human rights office report said there exaggerated, misinformed reports of attacks were used to create a climate of fear and insecurity. Although there were some attacks against the ethnic Russian community, these were neither systematic nor widespread, it said.
Russia called the report one-sided, politicised and apparently fabricated.