SLAVYANSK, Ukraine - At least one person was killed Sunday when Ukraine resorted to force to oust pro-Russian gunmen holed up in a police station in the restive east, as Moscow faced calls to defuse the crisis.
Interior Minister Arsen Avakov said a Ukrainian officer had died both sides had suffered casualties during the offensive in Slavyansk, which threatens to further escalate tensions with Moscow.
Russia has 40,000 troops massed on Ukraine's eastern border and has warned Kiev against using force.
"There are dead and wounded on both sides," Avakov wrote on his Facebook page, as unconfirmed Russian media reports said one protester had been killed.
"The separatists have started to protect themselves using human shields."
Helicopters hovered low over the poor mining town, where a thick column of black smoke could be seen, an AFP photographer said.
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.
Avakov earlier announced that units from "all of the country's force structures" were taking part in the first stiff response from Kiev to the unrest sweeping the volatile eastern part of the country.
With military precision and dressed in unmarked fatigues, unknown gunmen on Saturday launched a series of attacks against security buildings in the tinderbox eastern rust belt.
This came after a week of soaring tensions as pro-Russians demanding greater autonomy, or to join nearby Russia, stepped up protests in the region ahead of May 25 presidential polls.
The protesters refuse to recognise the new pro-Western government in Kiev, which swept to power on the back of bloody winter protests against fallen president Viktor Yanukovych's decision to reject closer ties with the European Union and move closer to Russia.
'A dangerous escalation'
Moscow has kept up crushing pressure on the new leaders, first seizing Crimea then threatening to cut off gas supplies and trade with the heavily indebted nation all while keeping up a massive military presence along the eastern border.
The West has expressed alarm that Russia is deliberately stoking tension in the heavily Russified east in order to justify a Crimea-style invasion.
"Militants in eastern Ukraine were equipped with Russian weapons and the same uniforms as those worn by Russian forces that invaded Crimea," US ambassador to Kiev Geoffrey Pyatt wrote on Twitter.
Avakov said the events were seen in Kiev as an "act of aggression" by Russia, which has flatly denied any role in the unrest sweeping Ukraine's east.
Britain's Foreign Office said the wave of occupations of government buildings was "a dangerous escalation".