Government forces repelled an attack Tuesday by about 30 gunmen at an airport in the first Ukrainian military action against a pro-Russian uprising in the east, according to the Associated Press.

The Ukrainian military acted against the pro-Russian uprising and kept an attack of about 30 gunmen at an airport, the AP reported.

The escalating conflict in Ukraine "essentially puts the nation on the brink of civil war," Russian President Vladimir Putin said on Tuesday, according to the AP. The president called the military action an "anti-terrorist operation" to try to restore authority over the restive region.

The escalating conflict in Ukraine "essentially puts the nation on the brink of civil war," Russian President Vladimir Putin said on Tuesday, according to the Associated Press.

His assessment came during a telephone conversation with his German counterpart, according to the Kremlin, the same day Ukraine's military launched its first, formal military action against pro-Russian militants with troops retaking an airport in the eastern Donetsk region after a reported clash with gunmen, the AP reported.

The military action came a day after a Ukrainian ultimatum expired for protesters to lay down their arms, a move that appeared to signal an escalation in the crisis that has sparked a diplomatic row between Ukraine, its Western allies and Russia, according to the AP.

With pro-Russian militants seizing government and police buildings in as many as 10 towns and cities in eastern Ukraine, acting President Oleksandr Turchynov told Parliament "an anti-terrorist operation" was under way in the region, the AP reported.

The aim of the military operation is to "stop attempts to tear Ukraine to pieces," he told lawmakers, according to the AP.

Witnesses reported hearing gunfire and aircraft that appeared to be coming from the airfield in Kramatorsk, which Turchynov's office said was under the control of Ukrainian special forces late Tuesday, the AP reported.

There were conflicting reports about casualties, with Russian state-run media citing varying casualty claims supplied by militants, according to the AP.

The clashes Tuesday came at Kramatorsk airport, just south of the city of Slovyansk, which has come under the increasing control of the pro-Russian gunmen who seized it last weekend, the AP reported.

The precise sequence of events was mired in confusion amid contradictory official claims, according to the AP. The commander of the Ukrainian operation, Gen. Vasyl Krutov, speaking outside Kramatorsk airport, said his men managed to thwart an attack by fighters in green military uniforms without insignia who tried to storm the facility in the late afternoon.