Ukraine has retaken control of the eastern city of Slavyansk including police buildings seized by pro-Russian militants on Saturday.
The Ukrainian interior ministry said one state security service officer has been killed and at least five others wounded in the so-called anti-terrorist operation.
On the side of the pro-Russian separatists there have been an “unidentifiable number” of casualties, according to the ministry.
Interior Minister Arsen Avakov said security units from across the country had been brought in to launch the “anti-terrorist operation” to re-assert Kyiv’s control in Slavyansk.
He appealed for people to leave the town centre, to stay in their apartments, away from the windows.
Meanwhile, groups of pro-Russian residents in Slovyansk have reportedly set up roadblocks around the city.