A pro-Russian supporter holds a Soviet flag in front of the seized office of the SBU state security service in Luhansk, in eastern Ukraine April 13, 2014. [Photo/Agencies]

KIEV - A security officer and a pro-Russia activist were killed Sunday during a raid launched by Ukrainian special forces in an eastern city, where pro-Russia gunmen seized police and security services buildings a day earlier.

"An anti-terrorist operation has begun in Slavyansk. It is being directed by the anti-terrorist center of the state security service. Forces from all the security units of the country have been brought in," Interior Minister Arsen Avakov said on his Facebook page.

He said the men who seized the buildings opened fire on the approaching troops, adding that a security officer was killed and five others wounded.

"There are deaths and injuries on both sides. On our side - an SBU (Ukrainian Security Service) officer. The head of the SBU's anti-terrorist center has been wounded, as have four others," he said. "On the side of the separatists - an unidentified number."

Avakov said the special forces had begun to regroup and urged local residents to remain calm and stay at home as a safety precaution.

"Pass this on to all civilians: they should leave the center of town, not come out of their apartments, and not go near the windows," he said.

Meanwhile, Russian news agency RIA reported that a pro-Russia activist was killed in clashes with government forces in Slavyansk. "On our side, another two were injured," local militant Nikolai Solntsev was quoted as saying.

Also on Sunday, protesters seized control of the mayor's office in Mariupol, another city in eastern Ukraine.

The protesters entered government buildings following a rally involving about 1,000 people demonstrating in favor of the creation of a separate republic in eastern Ukraine, local media reported.

The Ukrainian government's move in Slavyansk came one day after about a dozen gunmen seized a police headquarters and the Security Service office in the city amid simmering tensions in the country's Russian-speaking regions.

A new wave of unrest erupted in Ukraine's east last weekend, when pro-Moscow activists seized several government buildings in the cities of Donetsk, Lugansk and Kharkov, demanding a referendum on autonomy and closer ties with Russia.

On March 18, Russian President Vladimir Putin and Crimean leaders signed a treaty to make the southern peninsula part of Russian territory after almost 97 percent of Crimean voters backed secession from Ukraine in a referendum.

Kiev has rejected the referendum and Crimea's integration with Russia, saying it was unconstitutional.

Pro-Russian demonstrators announce Kharkov's independence	?Ukraine moves to end separatist grip on eastern town