Pro-Russian rebels shot down a Ukrainian helicopter in fierce fighting near the eastern town of Slaviansk today.

The government in Kiev has drafted special forces to the southwestern port city of Odessa to halt a feared westward spread of the rebellion.

Ukraine said the Odessa force, based on "civil activists", would replace local police who had failed to tackle rebel actions at the weekend.

Its dispatch was a clear signal from the Ukrainian government that, while tackling rebellion in the east, it would vigorously resist any sign of a slide to a broader civil war.

Odessa, with its ethnic mix from Russians to Ukrainians, Georgians to Tatars a cultural contrast to the pro-Russian east, was quiet today.

Ukrainian flags flew at half mast for funerals of some of the dozens killed in clashes on Friday.

But in the east, fighting intensified around the pro-Russian stronghold of Slaviansk, a city of 118,000, where rebel fighters ambushed Ukrainian forces early in the day.

The Interior Ministry said five Ukrainian paramilitary police were killed.

Separatists said four of their number had also been killed.             

The sound of an air-raid siren could be heard in the centre of Slaviansk, and a church bell rang in the main square.

Russia's foreign ministry called on Ukraine to "stop the bloodshed, withdraw forces and finally sit down at the negotiating table".

It also published an 80-page report detailing "widespread and gross human rights violations" in Ukraine over the past six months for which it blamed the new government and its Western allies.

Russia denies Ukrainian and Western accusations it is seeking to undermine the country of 45 million and using special forces to lead the insurgency across the border, as it did before annexing Crimea in March.

Self-declared pro-Russian mayor of Slaviansk Vyacheslav Ponomarev told Reuters by telephone: "[The Ukrainians] are reinforcing, deploying ever more forces here.

"Recently there was a parachute drop... For us, they are not military, but fascists."

Ukraine's Defence Ministry said rebels had shot down a Ukrainian military helicopter, the fourth since Friday, with heavy machine gun fire.

The helicopter crashed into a river and the crew were rescued alive, but there were no details of their condition.

The violence in Odessa marked a watershed for Ukraine.             

It increased fears that trouble could spread to the capital in the approach to Friday's celebrations of the Soviet victory in World War II, an event that could kindle tensions over the Kiev government's relations with its former communist masters in Moscow.

Over 40 people were killed in Friday's clashes, the worst since pro-Moscow president Viktor Yanukovych fled to Moscow in February amid protests by Ukrainians demanding closer ties to Europe.

Most were pro-Russians killed when the building they occupied was set ablaze by petrol bombs.

It is not clear who started the fire, but Russia accuses the government in Kiev of inciting violence.

Yesterday, hundreds besieged a police station where fellow pro-Moscow activists had been held since the shooting and fighting that led up to the house blaze.

Police then freed 67 of them, infuriating the Kiev government.

"The police in Odessa acted outrageously," Interior Minister Arseny Avakov wrote on his Facebook page. "The 'honour of the uniform' will offer no cover."

He said he had sent the newly formed Kiev-1 force of "civil activists" to Odessa following the sacking of the entire Odessa force leadership.

The units Mr Avakov referred to emerged partly from the uprising against Mr Yanukovych early this year.

That could fuel anger among the government's opponents, who accuse it of promoting "fascist" militant groups, such as Right Sector, that took part in the Kiev uprising over the winter.