Three pro-Russian protesters were killed and 13 injured during an attempted overnight raid on a Ukrainian National Guard base in the Black Sea port of Mariupol, Ukraine's authorities said.
The Interior Ministry said a mob of around 300 people armed with stun grenades and firebombs were involved in the bloodiest episode to date in the confrontation pitting Ukraine's new government in Kiev against an eastern insurgency tacitly supported by Moscow.
Masked and battle-ready militia bearing sophisticated firearms have been deeply involved in seizing government offices in eastern Ukraine, igniting suspicions that much of the unrest is being stirred with Russia's backing.
But Russia's President Vladimir Putin on Thursday dismissed as ''nonsense'' claims that Russian special forces were fomenting unrest in eastern Ukraine.
''It's all nonsense, there are no special units, special forces or instructors there,'' Putin said.
He did admit - for the first time - that the troops in unmarked uniforms who had captured Ukraine's Crimean Peninsula before its annexation last month by Moscow were Russian soldiers.
Putin also expressed hope that four-way talks between Ukraine, the U.S., the European Union and Russia in Geneva on Thursday could map a way out of the deepening security crisis.
Ukraine's interior ministry said shots fired by servicemen in the Mariupol base initially proved insufficient to deter the pro-Russian crowd from proceeding with their assault. It remains unclear exactly how the three protesters were killed.
There were no casualties among Ukrainian servicemen, the ministry said. At least 63 people involved in the attack were detained, but local media Thursday morning cited police as saying 38 were later released.
Speaking in parliament, acting President Oleksandr Turchynov said a pro-Russian gang carrying automatic weapons attempted to storm the base three times.
APTN footage filmed outside the base on Wednesday night showed an unidentified man acting as an intermediary coming out to speak to masked men in the crowd armed with assault rifles. He told them the military had asked for 10 minutes to think over an unspecified ultimatum.
The masked men insisted they wanted no bloodshed. A short while later, however, a crowd of mainly masked young men armed with bats and sticks began throwing Molotov cocktails at the base's gate and at the trucks parked in front of it. Sounds of gunfire were heard in response.
The skirmish came after the government announced an operation to retake control of the eastern city of Slovyansk,