Armoured vehicles bearing the Russian flag are reportedly rolling through the eastern Ukrainian town of Kramatorsk and the city of Slovyansk.
The development came as at least 20 pro-Russian protesters armed with Kalashnikov rifles seized the city council building and entered the mayor's office in the city of Donetsk, also in the east, a day after authorities in Kiev launched an operation to oust pro-Moscow separatists.
In Kramatorsk, dozens of armed men in camouflage were seated on top of the three vehicles as they drove through the city accompanied by a military truck on Wednesday, according to the Agence France Presse news agency.
The Associated Press was also reporting that six combat vehicles flying Russian flags rolled into Slovyansk, and were "enthusiastically greeted" at pro-Russian checkpoints outside the city.
The troops on those vehicles wore green camouflage uniforms, had automatic weapons and grenade launchers and at least one had the St. George ribbon - a symbol of the pro-Russian insurgency in eastern Ukraine - attached to his uniform.
One soldier, who identified himself only as Andrei, told AP the unit was part of the 25th brigade of Ukraine's airborne forces, and that they had switched to the side of the pro-Russian forces.
On Tuesday, Russian President Vladimir Putin warned that Ukraine was lurching towards a state of civil war, after Kiev began military operations in Kramatorsk and seized control of an airfield there.
The acting Ukrainian president, Oleksandr Turchynov, said his forces had taken the airfield in the first action of a threatened "anti-terrorist" campaign.
Videos showed military jets patrolling the skies around the airfield as the general in command of Ukraine's forces told pro-Russian crowds to disperse. Ukrainian soldiers were also seen stationed around the site.
The Reuters news agency reported that airborne forces had also landed in the nearby town of Slovyansk but were pulled back to base after meeting a hostile reception from local civilians.
Al Jazeera's Kim Vinnell, reporting from Kramatorsk on Tuesday, said military aircraft were sent from Izyum to bring Kramatorsk's airfield back under government control.
Itar-Tass, the Russian news agency, cited witnesses reporting that one of four Ukrainian military jets has been shot down. That was not confirmed by Ukrainian sources.
The developments came a day after a deadline set by Ukraine's acting Turchynov, calling on pro-Russian separatists to lay down arms and leave occupied government offices, expired on Monday at 0600 GMT.
The separatists, many of them armed, continued to occupy government, police and other administrative buildings in at least nine cities in the country's Russian-speaking east, demanding broader autonomy and closer ties with Russia.
Russia had earlier warned Ukraine against using force, saying that could prompt the country to walk out of Thursday's talks in Geneva.