There was heavy gunfire as armed men took the police station in Kramatorsk
Ukrainian forces and pro-Russian armed men have traded gunfire in a battle for control of the eastern town of Sloviansk, the interior minister says.
At least one Ukrainian officer was killed and both sides suffered casualties, Arsen Avakov said.
Pro-Russian forces took over the town on Saturday, prompting Kiev to launch an "anti-terror operation".
Kiev and Western powers accuse Moscow of inciting the trouble. The Kremlin denies the charge.
Sloviansk was one of three towns in the Donetsk region to be hit by pro-Russian gunmen on Saturday.
Russia's Foreign Minister Sergei Lavrov had warned earlier that any use of force in eastern Ukraine could scupper crisis talks due later this week.
He said the Kiev government was "demonstrating its inability to take responsibility for the fate of the country".
But the US said there had been a "concerted campaign" by forces with Russian support to undermine the authorities in Kiev.
US Secretary of State John Kerry warned of "additional consequences" if Russia failed to make efforts to "de-escalate" and pull its troops back from Ukraine's border.
Four-party talks involving Ukraine, Russia, the US and the EU are due to start in Geneva on Thursday.
On Saturday, armed men took over police stations and official buildings in Sloviansk, Kramatorsk and Druzhkovka.
Reports emerged from Sloviansk and Kramatorsk of armed men dressed in camouflage arriving in buses and storming the police stations.
Pro-Russian demonstrators also continued their occupation of the main administrative building in the regional capital Donetsk, which they have held for one week.
A protest leader told the BBC that the activists in Sloviansk took action to support the Donetsk sit-in.
BBC reporters in Sloviansk said the gunmen were well-organised and quickly established control throughout the town.
Checkpoints had been set up on the main roads into the town.
Mr Avakov labelled the actions a "display of aggression by Russia".
Announcing the operation to clear the activists, he warned people to stay in their homes in Sloviansk.
"The separatists are shooting to kill without warning against the approaching special forces," he said,
Local governor Sergei Taruta said "all legitimate means" would be used to combat "acts of terror" and restore order across Donetsk region.
Eastern Ukraine has a large Russian-speaking population and has seen a series of protests since the ousting of Ukraine's pro-Russian President Viktor Yanukovych in February.