Ukrainian forces launched a "large-scale operation" to retake the eastern town of Slaviansk, pro-Russian separatists holding the town said on Friday, as security deteriorated in a crisis that has provoked the biggest confrontation between Russia and the West since the Cold War.
A Reuters photographer said he saw a military helicopter open fire on the outskirts of the town and a reporter heard gunfire. Separatists said they were under attack and that at least one helicopter had been shot down.
Vyacheslav Ponomaryov, the self-declared mayor or the town, was quoted by Russia's Interfax news agency as saying two helicopters had been shot down and one pilot had been detained. Another had been killed.
Armed groups seeking union with Russia have seized a number of government buildings in towns in eastern Ukraine. The action in Slaviansk, if confirmed, would mark the first significant military response by Kiev.
In Kiev, an aide to Ukrainian Interior Minister Arsen Avakov said he could not comment. "Until it's over, no one will say anything," the aide said.
The apparent assault to retake Slaviansk came only hours after Russia staged a huge May Day parade on Moscow's Red Square on Thursday - its first since the Soviet era - with workers holding banners proclaiming support for President Vladimir Putin after the seizure of territory from neighboring Ukraine.