The acting president, Oleksandr Turchynov, said his forces had on Tuesday taken the airfield in Kramatorsk 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 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.
Later on Tuesday, Russia's president, Vladimir Putin, warned that "the sharp escalation of the conflict has placed the country, in effect, on the verge of civil war".
His comments came in a telephone conversation with the German chancellor, Angela Merkel, to discuss international negotiations on Ukraine in Geneva on April 17.
But the leaders both "emphasised the importance" of the talks, the Kremlin said in a statement. "The hope was expressed that the meeting in Geneva can give a clear signal to return the situation to a peaceful framework."
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 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.