Russia has said Ukraine is lurching towards a state of civil war after Kiev began military operations against pro-Russian separatists in the east of the country, seizing control of an airfield.
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 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.
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.
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.
"You can't send in tanks and at the same time hold talks. The use of force would sabotage the opportunity offered by the four-party negotiations in Geneva," said Sergey Lavrov, Russia's foreign minister.
But Ukraine found backing from Anders Fogh Rasmussen, NATO's secretary-general, who said in Luxembourg, where NATO ambassadors are due to meet on Wednesday, that Russia was meddling in the crisis in eastern Ukraine.
"From what is visible, it is very clear that Russia's hand is deeply engaged in this."