The escalating conflict in Ukraine "essentially puts the nation on the brink of civil war," Russian President Vladimir Putin said on Tuesday.
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 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.
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 aim of the military operation is to "stop attempts to tear Ukraine to pieces," he told lawmakers.
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 Ukrainain special forces late Tuesday.
There were conflicting reports about casualties, with Russian state-run media citing varying casualty claims supplied by militants. According to the reports, there were either two injured or four killed, claims that CNN cannot independently verify.