MOSCOW -- Ukrainian forces launched an "anti-terrorist operation" in the city of Slavyansk on Sunday morning after pro-Russian separatists seized buildings in the eastern area of the country, acting Ukrainian Interior Minister Arsen Avakov said on his Facebook account.

"Units from all law enforcement agencies of the country are being used [in the operation]," Avakov wrote. "God is with us!"

Armed men in masks and unmarked camouflage uniforms stormed and seized administrative buildings and police stations in Slavyansk and at least two other towns in the coal-mining region of Donetsk on Saturday. In Slavyansk, they captured scores of weapons in the police station and distributed them to local separatist activists, the UNIAN news agency reported. A Russian national flag was raised over the police station, the report said.

[Updated, 2:30 a.m. PDT April 13: In the course of the ongoing operation in Slavyansk there were casualties on both sides, including one law enforcement officer killed and five officers injured, Avakov said.