Army troops have restored control over a small airport in eastern Ukraine that had been occupied by pro-Russian militiamen, the acting president said.
It was the first military action taken by the government after Oleksandr Turchynov announced an "anti-terrorist operation" against the separatists who have seized control of numerous buildings in the east of the country.
Heavy gunfire rang out at the airport near Kramatorsk but there was no immediate confirmation of any casualties.
Russian state television reported four to 11 people had been killed, but there was no independent confirmation. Much of the shooting appeared to have been warning shots into the air.
Kramatorsk is just south of the city of Slovyansk, 160 kilometres (100 miles) from the Russian border, which has come under the increasing control of the gunmen who seized it last weekend.
Earlier in the day, an Associated Press reporter saw at least 14 armoured personnel carriers with Ukrainian flags, helicopters and military trucks parked 40 kilometres (24 miles) north of the city. Other heavy military equipment appeared nearby, along with at least seven busloads of government troops in black military fatigues.
"We are awaiting the order to move on Slovyansk," said one soldier, who gave only his first name, Taras.
Two of the helicopters loaded with troops were later seen taking off and flying towards Slovyansk. Witnesses said helicopters delivered several dozen troops to the Kramatorsk airfield.
The armed pro-Russian militias are occupying government, police and other administrative buildings in at least nine cities in the Russian-speaking east of the country around the major industrial city of Donetsk. They are demanding broader autonomy and closer ties with Russia.
The central government has so far been unable to rein in the insurgents, and many local security forces have switched to their side.
Speaking to parliament in Kiev, the capital, Mr Turchynov announced that the Kramatorsk airport had been secured.
"I'm convinced that there will not be any terrorists left soon in Donetsk and other regions and they will find themselves in the dock - this is where they belong," Mr Turchynov said.