Three pro-Russian protesters were killed and 13 were injured during an attempted overnight raid on a Ukrainian National Guard base in the Black Sea port of Mariupol, Ukrainian 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 the seizure of government offices in eastern Ukraine, igniting suspicions that much of the unrest is being stirred with Russia's backing.

Ukraine's Interior Ministry said shots fired by servicemen at 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 on Thursday morning cited police as saying 38 were later released.

Ukraine has banned entry to its territory for all Russian males aged between 16-60, Russian flagship carrier Aeroflot said on Thursday. "In line with an official order received by the company, all Russian male citizens aged from 16-60 will be refused entry into Ukraine," Aeroflot said, adding that exceptions would only be made in extreme cases.

AFP-AP