Two pro-Russian insurgents have been wounded in a skirmish with the Ukrainian military near Slavyansk, according to Russia's state-owned RIA-Novosti.
Kremlin mouthpiece RT also reported that several pro-Russian separatists were wounded in the attack. RT reported that "Other members of the militia have blocked the entrance to Kramatorsk and are ready to defend the city, the activists added."
However, the reports have not yet been confirmed independently.
Ukrainian forces are understood to have stormed an airfield between Kramatorsk and Slavyansk, in eastern Ukraine, and began firing on pro-Russian insurgents who had occupied the area, according to local reports.
Military helicopters and jet fighters were seen hovering over the airfield as Kiev announced an operation to retake it.
RIA-Novosti news agency said Ukrainian troops drove to the airfield in armored personnel carrier and started talking to the rebels who control the site. A skirmish erupted shortly after.
Russia's Prime Minister Dmitry Medvedev said that Ukraine was "on the brink of civil war" .
"Blood has once again been spilt in Ukraine," he said on his Facebook page.
A Reuters witness saw Ukrainian troops disembarking from two helicopters after landing in Kramatorsk.
Oleksandr Turchynov, the country's acting president, announced the launch of a military operation in the eastern Donetsk region to crack down on pro-Russian protesters who have seized government buildings.
The goal of the "anti-terrorist operation" is to protect Ukraine's citizens and stop criminal activity and attempts to tear the country apart, said Turchynov.