Ukrainian forces are said to have launched an operation to retake the town of Slovyansk, which has been in the hands of pro-Russian separatists.
Both sides say there have been casualties.
Ukraine’s Interior Minister Arsen Avakov says one helicopter pilot was killed and others wounded after pro-Russian forces fired anti-aircraft missiles.
“Against Ukraine’s special forces, terrorists used heavy artillery, including grenade launchers and portable anti-aircraft missile launchers. One pilot is dead and there are wounded,” Avakov posted on his Facebook page.
On the rebel side, the self-declared mayor of the town, Vyacheslav Ponomaryov, was quoted by Russia’s Interfax news agency as saying two Ukrainian military helicopters had been shot down, one pilot killed and another detained.
Earlier gunfire was heard, and a Reuters photographer saw a helicopter opening fire on the outskirts of the town, where pro-Russian forces have built barricades and set up checkpoints.
The scale of the operation is not clear but it does appear to be the first significant military response from Kyiv since pro-Russian rebels took control of government buildings in several towns across eastern Ukraine.