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 control several government buildings in town.
Armed pro-Russians also hold municipal buildings in Donetsk, another city where separatists demand referendums on splitting with Ukraine to join Russia.
Ukraine launched an operation Tuesday to try to take back the buildings.
Reports differ on whether shots were fired, if there are casualties, and who controls what.
NATO Secretary-General Anders Fogh Rasmussen says the alliance has decided on a series of immediate steps to reinforce its military footprint in eastern Europe, in response to Russia's moves.
"We will have more planes in the air, more ships on the water, and more readiness on the land. For example, air police and aircraft will fly more sorties over the Baltic region, allied ships will deploy to the Baltic sea, the eastern Mediterranean, and elsewhere as required."
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.