Armored personnel carriers with armed men riding on top and flying Russian flags rolled into the Ukrainian city of Sloviansk near the Russian border Wednesday.
Some of the men claim they are Ukrainian soldiers who have switched sides to the pro-Russians who already control several government buildings in town.
Also, armed pro-Russians have taken over the mayor's office and the city council building in Donetsk -- another city where separatists demand referendums on splitting with Ukraine to join Russia.
Ukraine launched what it calls an anti-terrorist operation Tuesday to try to take back the government buildings seized by the protesters.
There are different reports on whether shots were fired, if there are any casualties, and who controls what.
Russian President Vladimir Putin says the growing crisis in Ukraine has brought the country to the brink of civil war.
The Kremlin says Mr. Putin made his comment during a telephone call Tuesday with German Chancellor Angela Merkel.
President Putin accused the Ukrainian government of pursuing an "anti-constitutional" path by using force against the pro-Russian demonstrators who have taken over official buildings in 10 southeastern Ukrainian towns and cities.
The Kremlin says the Russian and German leaders hope Thursday's talks in Geneva between the European Union, Russia, Ukraine, and the United States will show the importance of finding a peaceful solution.