The Ukrainian Security Service said one of the helicopters was shot down with a surface-to-air missile, which it said undercut Russia's claims that the city is under control of civilians who took up arms. Stella Khorosheva, a spokeswoman for the pro-Russian militants, said one of their men was killed and another injured. She offered no further details. The official spokesman for the military wing of the pro-Russian forces, who will give only his first name, Vladislav, said fighting had broken out at several points around the city. He said government armoured vehicles were seen on roads leading into Slavyansk and claimed that Ukrainian troops had made incursions into the city itself. Details of these claims could not be independently confirmed. On the road leading into Slavyansk from Kramatorsk to the south, an Associated Press reporter saw six Ukrainian armoured vehicles parked on the side of the road.

An AFP reporter on the scene saw a column of eight armoured vehicles breaching a rebel-held checkpoint just south of Slavyansk.

The raid marked a dramatic escalation in the crisis and jeopardised negotiations to release seven European OSCE inspectors being held by Slavyank's insurgents.

Helicopters were seen circling low over Slavyansk, which has become the epicentre of rebel activity in the increasingly chaotic east of Ukraine.

One resident, Vladimir Pader, told Russian television: "Everyone - both rebels and Slavyansk residents - is determined not to surrender the city."

Eight armoured vehicles and several soldiers were seen taking position at a checkpoint just to the south of the town of 160,000 and warned off anybody trying to approach.

Sporadic small-arms fire and explosions were heard coming from the outskirts, while central Slavyansk remained relatively calm. Rebels had set at least one roadblock on fire to produce cloaking smoke.