Ukraine says its government forces are continuing an “anti-terror” operation in the eastern city of Sloviansk.
Interior Minister Arsen Avakov said one Ukrainian pilot had been killed, and other people wounded. Nine rebel checkpoints had been seized, he said.
However, separatists at three Sloviansk checkpoints told the BBC they were still in control there.
Pro-Russian gunmen have shot down at least two helicopters, reportedly capturing one pilot.
The fighting appears to be concentrating on the periphery of the city, says the BBC’s Sarah Rainsford in the regional capital Donetsk.
Sloviansk is a stronghold for pro-Russian separatists who are exerting increasing control in the region.
On Thursday, Russia’s foreign ministry warned that any assaults by Ukraine’s troops in the region would have “catastrophic consequences”, triggering fears of an invasion by Moscow.
He said interior troops and the National Guard were involved in the operation in the Sloviansk-Kramatorsk region.
“The terrorists opened fire with heavy weapons against Ukrainian special units.
“A real battle with professional mercenaries is going on,” Mr Avakov said, adding that the separatists were using the tactics of hiding behind civilians in residential buildings.
Russia’s state-run Rossiya 24 TV channel said the city was being “stormed”.