Ukraine's acting president, Oleksandr Turchynov, had earlier announced an "anti-terrorist operation" against the insurgents who have seized control of numerous buildings in the east of the country.
Heavy gunfire rang out at the airport near the town Kramatorsk. Russian state television reported four to 11 people had been killed, but there was no independent confirmation of any casualties. Much of the shooting appeared to have been warning shots into the air.
Kramatorsk is just south of the city of Slovyansk, 160 kilometers (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 armored personnel carriers with Ukrainian flags, helicopters and military trucks parked 40 kilometers (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 toward 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, Turchynov announced late Tuesday 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," Turchynov said.
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