Dozens of Ukrainian soldiers have also been brought in to Slavyansk where pro-Russian separatists have seized the local headquarters of the police and state security service.
The operation has significantly raised fears that Moscow might now intervene militarily on behalf of Russian speakers in the region.
Moscow has reportedly amassed thousand of troops along the border with Ukraine.
Writing on his Facebook page, Russia’s Prime Minister Dmitry Medvedev described Ukraine as being on the “brink of civil war.”
Moscow accuses Kyiv of provoking the crisis by ignoring the rights of citizens who use Russian as their first language, and has promised to protect them from attack.
Russian Foreign Minister Sergei Lavrov denied that Moscow was stirring up the separatists in the east and southeast as a possible prelude to repeating its annexation of Crimea. “Ukraine is spreading lies that Russia is behind the actions in the southeast,” Lavrov said on a visit to China.
Kyiv opposes anything that might lead to the dismemberment of the country. But in an attempt to undercut the rebels’ demands, Turchynov has held out the prospect of a nationwide referendum on the future shape of the Ukrainian state.