Reports say the Ukrainian military has launched an offensive to take back the main airport in Donetsk following its seizure by armed separatists.
Vladislav Seleznyov, a spokesman for Kyiv's counterterrorist operation, said the military launched an air strike on gunmen occupying the airport after an ultimatum to them to leave expired.
Donetsk has provided a major foothold for militant pro-Russian separatists.
Western media reported the sounds of explosions and shooting at the airport, with smoke billowing from inside its perimeter.
Explosions and heavy machine-gun fire could be heard from the area, and plumes of black smoke billowed into the sky.
A military spokesman said Mi-8 helicopters along with SU-25 fighter jets and MiG-29s were being used in the operation.
Reports also said that several trucks arrived at the airport carrying scores of pro-Russian gunmen.
The airport shut down earlier in the day after armed men demanded that soldiers guarding the perimeter of the airport be withdrawn.
PHOTO GALLERY: Scenes from the fighting in Donetsk
The development comes as the leader of the separatist, self-declared Donetsk People's Republic said he was imposing martial law in Ukraine's Donetsk region.
Denis Pushilin said in a tweet that martial law went into effect after midnight.
There were reports that the separatists had ignored a deadline to hand over their weapons and surrender to Ukrainian military forces.
Russia's Interfax quoted Vladyslav Seleznyov, a security forces spokesman, as saying via Facebook: "At 13:00 the terrorists who had illegally penetrated onto the territory of the airport in Donetsk were issued an ultimatum for them to lay down their arms and surrender to the counterterrorist operation forces. It has been reported that if the demands were not satisfied the Ukrainian military would take the military option to liberate the airport."
Based on reporting by AFP and Reuters