Ukrainian government troops clashed with armed activists at the airport in Ukraine's eastern city of Donetsk Monday, one day after the presidential elections were held in the country.
Heavy black smoke billowed after fighter jets were seen flying over and explosions were heard, Xinhua reported.
The Ukrainian government said a military operation had begun at Donetsk's international airport.
"A full-scale anti-terrorist operation is under way at the airport," a source told local news website Ostrov.
Armed activists in the eastern Donetsk and Lugansk regions established the "people's republic" in early April and an overwhelming majority of voters favoured their regions' independence at referendums held May 11.
Kiev has rejected the referendums as illegal and launched "anti-terrorist operations" in the regions.
Petro Poroshenko, the likely winning presidential candidate, told a news conference in Kiev that he would not negotiate with terrorists in eastern Ukraine.
Earlier in the day, eyewitnesses said they heard explosions near the airport after armed men arrived there.
The Donetsk airport's press wing said the airport had been closed after militants of the self-proclaimed Donetsk People's Republic arrived, demanding the withdrawal of Ukrainian forces guarding the airport.
