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Sarah Rainsford in Donetsk says the scale of clashes on the ground is not yet clear
Pro-Russian rebels have shot down two of Ukraine's army helicopters during an "anti-terror" operation in the eastern city of Sloviansk, Kiev has said.
It said a pilot and serviceman had been killed, four suspected separatists held and 10 rebel checkpoints seized.
Half of the city was later declared "under control" of the Ukrainian units.
There has been no independent confirmation of the claim. Separatists at three checkpoints earlier told the BBC they were still in control there.
Russia said the use of the army by Kiev against its own people "is leading Ukraine to catastrophe".
Describing the military operation as "punitive", the Russian foreign ministry also urged Western powers to give up their "destructive" policy on Ukraine.
The ministry earlier warned that any assaults by Ukraine's troops in the region would have "catastrophic consequences", triggering fears of an invasion by Moscow.
Separately, several foreign journalists were reportedly detained in Sloviansk and some of them were released later.
Sloviansk is a stronghold for pro-Russian separatists who are exerting increasing control in the region.