Pro-Moscow youths have recaptured a government building in Donetsk just hours after Ukrainian authorities managed to fly their own flag there for the first time since Saturday.
The offices had been under control of pro-Russian forces since Monday. Now the Russian flag flies from the roof once again.
The Crimean crisis has raised international tensions and sparked fears of war.
Forces believed to be Russian have surrounded the main military bases in Crimea. The Ukrainian authorities confirmed two missile defence bases have also been encircled.
The border guard service had said Russian navy ships have blockaded both ends of the Kerch Strait between Crimea and Russia, although civilian shipping waterways remained open.
Russia says the ships in Crimea were part of the Black Sea fleet.