Ukrainian security forces have launched an "anti-terrorist" operation to end the grip of separatists on an eastern town, according to the country's interior minister.
One Ukrainian security officer has been killed, and five others injured since the operation began early on Sunday.
At least one pro-Russian activist was also killed, Reuters news agency reported.
"An anti-terrorist operation has begun in Slovyansk. It is being directed by the anti-terrorist centre of the state security service. Forces from all the security units of the country have been brought in," Interfax news agency quoted the interior minister, Arsen Avakov, as saying on his Facebook page on Sunday.
However, details of the fighting remain sketchy.
A statement from the administration of the eastern Donetsk region indicated the security officer may have been killed between Slovyansk and the nearby town of Artemivsk.
Putting the number of wounded at nine, it said "an armed confrontation" was going on in the area.
Pro-Russian activists carrying automatic weapons seized government buildings in the town about 150km from the Russian border on Saturday, and set up barricades on the outskirts of Slovyansk.
Avakov also reportedly warned residents of the city to stay indoors, in anticipation of clashes between the gunmen and Ukrainian security forces.
"Pass it on to all civilians: they should leave the centre of town, not come out of their apartments, and not go near the windows," Avakov was quoted as saying.
Outside the town's police headquarters, one of the buildings seized, two military helicopters were flying overhead, according to Reuters news agency.
About a dozen women, who had been in the building when it was seized, were evacuated as soon as Avakov's announcement was made known.
A large group of civilians, many of them elderly women, stood in front of the building, chanting messages of solidarity to the protesters inside, including "Referendum!", a reference to demands for a local poll to be held to determine a separate status for the region.
On the building's forecourt, which itself was barricaded, protesters beat out a defiant tattoo on shields they were carrying.
Ukraine accuses Russia of trying to deepen violence and chaos in Ukraine, a former Soviet republic it once ruled. The Kremlin, it says, wants to undermine the legitimacy of presidential elections on May 25 which aim to set the country back onto a normal path after months of turmoil.
However, Sergey Lavrov, Russian foreign minister, said Ukraine was "demonstrating its inability to take responsibility for the fate of the country", and cautioned that any use of force against Russian speakers "would undermine the potential for cooperation", including talks due to be held on Thursday between Russia, Ukraine, the US and the EU.