Ukraine is launching a "large-scale anti-terrorist operation" to resist attacks by armed pro-Russian forces in the east of the country, President Oleksandr Turchynov said yesterday in a televised address to the nation.
This came after Ukrainian special forces exchanged gunfire with a pro-Russia militia in an eastern city of Slavyansk that left at least one Ukranian officer dead.
Turchynov said the authorities in Kiev would use the army to prevent Russian forces from moving into Slavyansk, as they did in Crimea, pledging amnesty to anyone laying down arms by this morning.
"Blood has been spilt in a war that is being waged against Ukraine by Russia," Turchynov said.
He said a Security Service captain was killed and two colonels wounded in a gunbattle outside Slavyansk, where the police station and the Security Service office were seized a day earlier.
Interior minister Arsen Avakov said in a Facebook post that there were an "unidentifiable number" of casualties among the separatists. He called on residents to remain calm and to stay at home.
Residents, mostly women, huddled in the cold under light rain in front of barricades protecting the police building. Armed separatists have also set up a checkpoint at the entry to the town.
Watch: Pro-Russian gunmen dig in positions at occupied police station
Unrest has spread to several municipalities in eastern Ukraine, including the major industrial city of Donetsk, which has a large Russian-speaking population.
Donetsk was also the support base for Viktor Yanukovych, the Ukrainian president ousted in February following months of protests in Kiev, the capital.
Ethnic Russians in Ukraine's east widely fear that the new pro-Western Ukrainian government will suppress them.
The regional administration in Donetsk issued a statement, confirming one dead but said there were altogether nine people wounded. They wouldn't say who the wounded people were.
Russian state-run Rossiya 24 television said Ukrainian "self-defence" forces led by an Afghan war veteran had spread across Slavyansk, while troops allied to the government in Kiev arrived in armoured personnel carriers and by helicopter.
Moscow has kept up crushing pressure on the new leaders, first seizing Crimea then threatening to cut off gas supplies to the heavily indebted nation while keeping up a massive military presence along the eastern border.
Additional reporting by Reuters, Agence France-Presse