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airport shut down earlier in the day after armed men demanded that soldiers guarding the perimeter of the airport be withdrawn.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
Pro-Russian separatists of the self-proclaimed "Vostok Battalion" preparing to board a truck at a checkpoint on the outskirts of the city of Donetsk on May 25.
Pro-Russian separatists said to be from the self-proclaimed "Vostok Battalion" cheered on their comrades aboard a truck at a checkpoint on the outskirts of Donetsk on election day, one day before the airport fighting.
A military truck with heavily armed pro-Russian militants drives through a police checkpoint toward the airport of the eastern city of Donetsk.
Pro-Russian militants patrolling Donetsk's international airport early on May 26, before the heavy fighting erupted.
A pro-Russian militant takes up a position atop the Donetsk airport's roof.
The militants set up a mortar launcher atop Donetsk's international airport.
The terminal of Donetsk international airport is seen behind a cemetery.
Pro-Russian militants on the roof of the airport.
A separatist gunman takes up a position inside the airport ahead of the heaviest fighting.
Smoke rises from Donetsk's international airport during heavy fighting between Ukrainian and pro-Russian forces on May 26.
Smoke rises through a cemetery over the Donetsk airport during fighting between Ukrainian forces and pro-Russian separatists.
Smoke billows from Donetsk airport during heavy fighting between Ukrainian and pro-Russian forces.
A Ukrainian Mi-24 helicopter gunship fires its cannons at rebels at the main terminal building of Donetsk's international airport on May 26.
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