Pro-Russian militants have extended their takeovers of public buildings in eastern Ukraine.
The regional interior ministry says gunmen have seized a state security building in Slovyansk.
Ukraine's foreign minister has urged Russia to stop "provocative" actions.
The Russian government has denied providing support to the militants.
Earlier Saturday, armed men seized a police station in the city. A VOA correspondent in the region says the militants took about 400 weapons from the station.
Meanwhile, pro-Russian protesters continue to occupy government buildings in the regional capital Donetsk and in Luhansk. On Saturday, protesters armed with bats stormed a government building in Donetsk, where they were confronted by police.
Kyiv has offered concessions to the protesters and regional leaders, after its Friday deadline passed for separatists to vacate the buildings.
Acting Prime Minister Arseniy Yatsenyuk said he supported amending Ukraine's constitution and changing laws so regional governors are no longer appointed by the central government, and regional referendums are permitted. He also promised no one would be allowed to "limit the Russian language and the right to speak it in Ukraine."
NATO says there has been a buildup of Russian military forces along the border with Ukraine recently.
Saturday there were signs of heightened tensions between Moscow and Kyiv. Officials at Ukraine's state-run energy company, Naftogaz, said the company was suspending gas payments to Russia.
Russia announced earlier it was increasing its price for gas to Ukraine.
On Friday, Russian Foreign Minister Lavrov said he is ready for four-party talks on Ukraine with the U.S., the European Union and Kyiv. Lavrov also said Washington should use its influence with Ukraine's government "to prevent the use of force" and encourage dialogue.
Recent surveys suggest residents of eastern Ukraine overwhelmingly oppose any move to join Russia.
The U.S. is threatening to impose more economic penalties on Moscow if Russia does not pull troops back from the border with Ukraine.