Ukraine's interior minister said on Sunday that both sides had suffered casualties during a raid launched by Ukrainian special forces on a police station in the eastern city of Slavyansk that was seized by pro-Russian gunmen. "There are dead and wounded on both sides. On our side -- an SBU (Ukrainian Security Service) officer. The head of the SBU's anti-terrorist centre has been wounded, as have four others," Interior Minister Arsen Avakov wrote on his Facebook page. "On side of the separatists -- an unidentified number. The separatists have started to protect themselves using human shields." Avakov added that Ukraine's special forces have begun to "regroup" but he gave no other details. About 20 pro-Kremlin gunmen on Saturday seized the Slavyansk police station and later occupied the city's SBU security service building. The raids were accompanied by unconfirmed reports of police stations in other nearby cities in the heavily Russified east of the country also falling under gunmen's control. Video footage aired on Ukrainian television showed gunmen opening fire on Saturday during a raid on a police station in the nearby town of Kramatorsk. Ukraine's interior ministry had on Saturday denied that the Kramatorsk police station had also been taken by the armed separatists. Avakov only said in his Facebook posting that "the people of Kramatorsk are unhappy with the actions of the invaders". The east of Ukraine has been rocked by protests demanding that the region stage referendums on joining Kremlin rule similar to the one that led to Crimea's annexation by Russia last month.