Russia has called an emergency UN Security Council meeting after at least seven people were killed in an Ukrainian military assault on the flashpoint town of Slavyansk, the deadliest day for months in the escalating crisis.

A furious Kremlin said the raid was “leading Ukraine towards catastrophe” and pronounced dead a peace deal struck in Geneva last month to ease the worst East-West confrontation since the Cold War.

Russian Prime Minister Dmitry Medvedev urged the western-backed leaders in Kiev to “stop killing their citizens”, saying the raid was “a sign of criminal helplessness”.

In a day of violence that dramatically escalated the Ukraine crisis, one person was also shot dead in the southern port city of Odessa, as pro-Russian militants clashed with supporters of Ukrainian unity.

Ukraine’s interim president Oleksandr Turchynov said “many rebels” had been killed in the pre-dawn raid on the eastern town of Slavyansk and confirmed the loss of two military personnel after insurgents shot down two helicopters.

Rebels later said three of their number and two citizens were killed in what they said was a “full-scale attack” and vowed to defend the town, which has become the epicentre of tension in increasingly volatile eastern Ukraine.

As the raid began, reporters saw a column of armoured vehicles breaching a rebel-held checkpoint just south of Slavyansk and heard explosions and sporadic small arms fire as helicopters circled overhead.

Kiev said its military had taken over nine rebel checkpoints and the army appeared to entrench their positions after the initial assault, tightening their encirclement of the flashpoint town.

The attack seemed to dash hopes of a quick freeing of seven European monitors being held in Slavyansk, with one senior rebel leader saying it would “delay” their release.

Turchynov has already reintroduced conscription in Ukraine, amid fears the Kremlin could send in its estimated 40,000 troops parked on the border.

He has ordered his army on “full combat alert” after admitting authorities are “helpless” to prevent pro-Russian militias extending their grip on more than a dozen towns and cities.