A member of the Ukrainian special forces stands guard outside the regional administration building in Kharkiv on Tuesday. Ukraine has launched an “anti-terrorist” operation in the eastern city of Kharkiv and about 70 “separatists” have been arrested for seizing the regional administration building, Ukrainian Interior Minister Arsen Avakov said on Tuesday. OLGA IVASHCHENKO / REUTERS

Russia warned Kiev on Tuesday that any use of force in Ukraine's east, where pro-Kremlin militants have seized government buildings in several cities, could tip the country into civil war.

"We call for the immediate cessation of any military preparations, which are fraught with the risk of unleashing civil war," Russia's Foreign Ministry said in a statement.

The warning came as Ukrainian authorities released details of an "anti-terrorist" operation in the eastern city of Kharkiv that was aimed at capturing pro-Russian activists who had occupied a public building there.

Police detained 70 people occupying a regional administrative building on Monday night, bringing the building under government control.

Ukrainian special forces in combat gear, helmets and balaclavas and carrying kalashnikovs and machine guns stood guard early on Tuesday outside the building, whose outside windows were broken.

A partly destroyed sign near the main door read, "Avakov - to jail", a reference to Ukrainian Interior Minister Arsen Avakov.

On Sunday, pro-Russia protesters seized state buildings in the eastern cities of Kharkiv, Lugansk and Donetsk, where they also declared independence and vowed to hold a referendum on joining Russia.

Kiev accused Russia of fomenting the unrest, and Washington warned the Kremlin to stop efforts to "destabilize Ukraine", accusations that Moscow brushed off.

Kiev described the latest unrest as a replay of events in Crimea, the Black Sea peninsula that Moscow annexed last month after a referendum there.

Forces deployed

The Russian Foreign Ministry said on Tuesday it had information that Ukraine was sending internal security forces and volunteers from its National Guard, including fighters from the Pravy Sektor (Right Sector) ultra-nationalist group, to southeastern Ukraine, including Donetsk.

It also alleged that Ukraine was deploying US private security operatives dressed as Ukrainian special forces. It said the mercenaries came from the Greystone Ltd security firm.

It said Ukraine had tasked the forces with "suppression using force" of the residents in the southeast of the country who oppose the policies of the government in Kiev.

"The organizers and participants in this provocation are taking on a huge responsibility for creating a threat to the rights, freedoms and lives of peaceful Ukrainian citizens and to the stability of the Ukrainian state," the Russian ministry warned.