Ukraine's acting president has called an emergency meeting, after pro-Russian militants seized more buildings, on Saturday, and set up military style checkpoints.
President Oleksandr Turchynov announced he would meet with his national security council, late Saturday.
Earlier, Interior Minister Arsen Avakov said Ukraine considered Saturday's unrest an "act of aggression" by Russia.
He commented after armed pro-Russian militants expanded their take-overs of government buildings in eastern Ukraine, including the police headquarters in the regional capital, Donetsk.
A VOA correspondent at the scene said heavily armed gunmen wearing uniforms took over the building, in support of pro-Russian demonstrators. The gunmen said they would secure the weapons arsenal in the building. The seizure prompted the regional police chief to resign.
Pro-Russian militants also seized a state security building in Slovyansk and occupied a building in Luhansk.
Officials also say armed gunmen attacked a police station in the town of Kramatorsk and set up checkpoints along roads.
Ukraine's foreign minister urged Russia to stop "provocative" actions.
The Russian government has denied providing support to the militants.
There were other signs of heightened tensions between Moscow and Kyiv, on Saturday. 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.
Ukraine's current government came to power following the so-called "Euromaidan" protests that forced the country's pro-Russian leaders from power in February.
A month later, Moscow proceeded to annex Ukraine's Crimean peninsula, a move that prompted Washington to impose sanctions on Russia.
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.