Shooting was reported at Kramatorsk airport.
Russia's Interfax news agency quoted a witness as saying the Ukrainian military had taken control of the airport. No other confirmation was available.
There were reports of some casualties as a result of what was described as a battle.
The Reuters news agency cited a witness as saying two helicopters landed and Ukrainian troops disembarked at the airport.
The Russian television channel Rossiya 24 showed footage from Kramatorsk that included warplanes flying over the airport and helicopters hovering over the area.
Elsewhere, Ukrainian forces backed by armored vehicles reached the outskirts of Slovyansk, another town where a pro-Russian group has seized power.
The appearance of the Ukrainian forces comes one day after acting President Oleksander Turchynov vowed to use force if necessary to assert Ukrainian government control over at least 10 cities in eastern Ukraine where pro-Russian groups have seized administrative buildings.
Most dangerous moment yet as #Ukraine forces try to retake town of #Kramatorsk from pro #Russia separatists. Rumour & misinformatn flying.
— Lindsey Hilsum (@lindseyhilsum) April 15, 2014
Have to say lots of contradictory reports coming in and no one can say for sure what has or hasn't been taken, etc
— bruce springnote (@BSpringnote) April 15, 2014
Locals have set up a barricade outside Kramatorsk airfield. Anti-terrorist operation commander Krutov talking to angry crowd
— Alec Luhn (@ASLuhn) April 15, 2014
Ukrainian army general is addressing people in #Kramatorsk airport. The crowd is getting angry
— Petr Shelomovskiy (@stopnarcotics) April 15, 2014
#Ukraine troops storm #Kramatorsk airfield, #Sloviansk; unconfirmed reports of casualties (LIVE UPDATES) http://t.co/dak99BA6w8 @KyivPost
— Christopher Miller (@ChristopherJM) April 15, 2014
Anti-terrorist operation finally started in Eastern #Ukraine. Separatist-controlled airfield in Kramatorsk stormed,APCs surrounded Sloviansk
— Olga Tokariuk (@olgatokariuk) April 15, 2014
!!! BREAKING NEWS !!!
This just in from our news desk:
Shooting has been reported at the Kramatorsk airport.
Russia's Interfax news agency cited the head of the pro-Russia Donbas volunteer group, Sergei Tsyplatkov, as saying two members of his force have been wounded.
The Reuters news agency cites a witness as saying two helicopters landed and Ukrainian troops disembarked at the airport.
Russian paramilitary leaders in eastern #Ukraine caught on tape communicating with #Moscow http://t.co/EUidXBgxIW pic.twitter.com/vRKiXlmQJ0
— Christopher Miller (@ChristopherJM) April 15, 2014
"Trigger Happy" - Russian TV says "tanks from Kiev" are on the move in eastern Ukraine: pic.twitter.com/THMHVsQByr
— Mark MacKinnon (@markmackinnon) April 15, 2014
Correspondents for the Associated Press and the AFP news agencies reported seeing a column of armored vehicles and tanks north of Slovyansk on April 15.
The appearance of the Ukrainian forces comes one day after acting President Oleksander Turchynov vowed to use force if necessary to assert Ukrainian government control over at least 10 cities in eastern Ukraine where pro-Russian groups have seized administrative buildings.
Turchynov has condemned as "terrorists" the pro-Russian groups occupying government buildings and security headquarters in eastern Ukraine.
The Russian ITAR-TASS news agency cited the pro-Russian acting mayor of Slovyansk, Vyacheslav Ponomaryov, as saying the city is surrounded by Ukrainian government forces.
He said "self-defense militias" were reinforcing barricades at entry points to the city.