Russian President Vladimir Putin warned that Ukraine was on the verge of civil war Wednesday, after the Kiev government sent in troops against pro-Russian demonstrators in the east of the country.

But Nato responded with hostility, announcing that it was strengthening its military footprint immediately.

Secretary-general Anders Fogh Rasmussen said Nato aircraft would fly more sorties over the Baltic region and allied warships would deploy to the Baltic Sea, the eastern Mediterranean and elsewhere if needed.

"We will have more planes in the air, more ships on the water and more readiness on the land," he said.

Ukraine had pushed tanks towards the eastern city of Slovyansk on Tuesday, in the most forceful response yet by the Western-backed coup government in Kiev to the pro-Kremlin militants' occupation of state buildings in around 10 cities across Ukraine's rust belt.

"They must be warned that if they do not lay down their arms, they will be destroyed," Ukrainian Security Service General Vasyl Krutov said.

However, yesterday, a column of armoured personnel carriers flying Russian flags drove into Slovyansk.

The vehicles stopped near the city administration building while residents chanted "Good job, Good job."

One of the men who arrived in the vehicles, who identified himself only as Andrei, said the unit was part of Ukraine's 25th Brigade of Airborne Forces and that they had switched to the side of the pro-Russian forces.

Journalists also reported local people challenging Ukrainian soldiers, who had also been confronted by another crowd of civilians outside Kramatorsk.

One officer reassured the crowd that he had not "come to fight" and would never obey orders to shoot his "own people."

Insurgents in Slovyansk have seized the local police headquarters and administration building, demanding broader autonomy for the region and closer ties with Russia.

And in Donetsk, about 20 pro-Russian protesters armed with Kalashnikov rifles entered the mayor's office.