One Ukrainian state security officer was killed and five others were wounded on the side of government forces in an "anti-terrorist" operation on Sunday against pro-Russian separatist militants in a city in the east, the interior minister said.

On the side of the separatists there had been an "unidentifiable number" of casualties during the operation in the town of Slaviansk, the minister, Arsen Avakov, said on his Facebook page

Ukrainian security forces launched the operation earlier Sunday to end the grip of separatists on an eastern town.

"An anti-terrorist operation has begun in Slaviansk. It is being directed by the anti-terrorist center of the state security service. Forces from all the security units of the country have been brought in," Interfax news agency quoted , Ukraine's interior minister, Arsen Avakov, as saying on his Facebook page.

Avakov told residents in Slaviansk to stay indoors, in anticipation of clashes between pro-Russian militants who have seized official buildings 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," Interfax Ukraine news agency quoted Arsen Avakov as saying.

He also said there was gunfire in the city, but a Reuters reporter outside the town's police headquarter in Slaviansk said he had not heard shooting, but did say that two military helicopters were flying overhead.

Pro-Russian activists carrying automatic weapons seized government buildings in the town about 150 kilometers (90 miles) from the Russian border on Saturday, and set up barricades on the outskirts of the city.

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 group of up to a 100 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.

Open gallery view Pro-Russian protesters seize the police headquarters, while a military helicopter flies above, in Slaviansk April 13, 2014. Credit: Reuters