Six armoured personnel carriers have been seized by pro-Russian armed insurgents who then drove them through the centre of the eastern Ukrainian town of Kramatorsk with the lead vehicle bearing the Russian flag, according to eyewitnesses.

Several armed men, numbering about 100 according to observers, in balaclavas and wearing military uniforms were seen on top of the seized vehicles, which stopped outside the town's city hall occupied by the pro-Russian rebels. As they passed by, the vehicles were enthusiastically greeted and cheered by the crowd at pro-Russian checkpoints outside the city.

A 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 have switched to the side of the pro-Russian forces.

The incident occurred when at least 20 pro-Russian separatists took control of the city council building in the eastern Ukraine city of Donetsk.

Ukraine forces launched a military operation in the east of the country on Tuesday, storming an airfield between Kramatorsk and Slavyansk and began firing at pro-Russian insurgents who had occupied the area, according to local reports.

As Kramatorsk airfield was being seized, Ukrainian units were also gathering outside the nearby city of Slavyansk in an operation to free government buildings from armed pro-Russian groups.

"I just got a call from the Donetsk region: Ukrainian special forces have liberated the airport in the city of Kramatorsk from terrorists," Oleksandr Turchynov told parliament. "I'm convinced that there will not be any terrorists left soon in Donetsk and other regions and they will find themselves in the dock – this is where they belong."

Pro-Russian protesters calling for independence from Kiev rule have occupied at least nine government buildings in the region for more than a week. However, this is the first time pro-Russian forces have seized military arsenal in the area.