UKRAINE’S Kiev government today launched its first major military assault against pro-Russian activists who have seized government buildings in the country's east.

Fighting broke out in the early hours around the city of Sloviansk.

Three deaths were reported in early fighting, including a government helicopter pilot.

Two Ukrainian helicopters were shot down in morning actions.

Stella Khorosheva, a spokeswoman for the pro-Russian militants, said that one of their men had been killed and another injured.

Russian President Vladimir Putin’s spokesman said that the use of the military by the Ukranian government against “its own people” had “effectively destroyed the last hope for the implementation of the Geneva agreements.”

Putin spokesman Dmitry Peskov said the Kremlin had sent an envoy to negotiate the release of foreign military observers who were captured by pro-Russian militia in Sloviansk.

But he said the Kremlin had not been able to get in touch with envoy Vladimir Lukin since Ukraine launched the offensive. 

News agencies later quoted Mr Lukin’s aides as saying he was in touch and safe.

The security service claimed that its forces were fighting “highly skilled foreign military men” in Sloviansk, adding that one helicopter had been shot down with a surface-to-air missile which they said countered Russia's claims that the city is under control of civilians.

Ukraine Interior Minister Arsen Avakov said that government troops met fierce resistance, but had managed to take control of nine checkpoints on roads around Sloviansk.

Avakov called on pro-Russian activists to lay down the arms and release their hostages.

“We are ready to negotiate with protesters and their representatives,” Mr Avakov said. “But for terrorists and armed separatists there is only punishment.”

A pro-Russian activist said fighting had broken out at several points around the city and it was now effectively surrounded. 

By late afternoon the security service reported that half of Sloviansk was under the control of the Ukrainian army — a claim that could not be immediately verified.