Pro-Russian supporters attend a rally in front of the seized office of the SBU state security service in Luhansk, eastern Ukraine April 12, 2014.[Photo/Agencies]

SLAVIANSK, Ukraine - Ukrainian security forces have launched an "anti-terrorist" operation to end the grip of separatists on an eastern town, Ukraine's interior minister said on Sunday.

"An anti-terrorist operation has begun in Slaviansk. 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 minister, Arsen Avakov, as saying on his Facebook page.

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

A Reuters correspondent outside the town's police headquarters, one of the buildings seized, said two military helicopters were flying overhead.

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.