Ukrainian armed forces on Tuesday launched a "special operation" against separatists in the town of Kramatorsk in the east of the country, the defence ministry was quoted as saying by Interfax.
Local journalists quoted by Interfax said automatic firing could be heard from the direction of Kramatorsk's military airfield and said a fighter jet had swooped low over the area.
A correspondent of Espresso TV meanwhile said the Ukrainian plane had been trying to land but had not been able to do so because separatists had opened fire on it.
Earlier, pro-Russian militants who had been holed up in the police headquarters since Saturday had quit the building - but a state security official in Kiev said separatists had then taken over the agency's offices in the town.
A Reuters correspondent in the town reported seeing first a fighter jet over the town and then four military helicopters over the airport.
Two of these landed and when troops stepped out and walked across the field, locals manning a barricade shouted "Shame! Go back home!"
The correspondent, Gabriela Baczynska, said locals with Russian flags had set up barricade of sand and tyres outside the gates to the airport. Some of them appeared to be preparing petrol bombs.