Ukrainian acting Interior Minister Arsen Avakov has called on the residents of Slaviansk to not leave their homes or stay near windows due to an ongoing "anti-terrorist operation."

Avakov warned about clashes on Facebook, announcing that separatists had opened fire on special forces without negotiations in the eastern town of Slaviansk, in Ukraine's Donetsk region.

Earlier the same day, Ukraine's interim Interior Minister informed that security forces had launched an "anti-terrorist operation" Sunday, after pro-Russian gunmen seized buildings in the eastern part of the country.

On Saturday, unidentified pro-Russian gunmen in camouflage stormed and seized a police headquarters in Slaviansk.

The gunmen introduced themselves as part of the Donetsk republic initiative group, according to reports of local police, as cited by the CNN.

Protesters in Donetsk have been occupying government buildings for days and demanding a referendum on becoming part of Russia.