Slavyansk - Gunfire and heavy detonations were heard on Friday just outside Ukraine's rebel-held town of Slavyansk, in what the insurgents said was "a full-scale attack" by the Ukrainian military.
A helicopter was earlier seen circling low, said the rebels, who had set a roadblock on fire to produce cloaking smoke.
"This is a full-scale attack," said the spokesperson in the town for the insurgents.
She said the rebels had shot down a helicopter.
Journalists in the town were attempting to verify the information after hearing the heavy detonations just after dawn.
An AFP reporter said the action appeared localised.
Slavyansk itself was relatively calm, but the town's church was ringing its bells to alert the population of 160 000.
The rebel leadership of the town, however, has been warning for several days that the Ukrainian military was preparing an assault.
Slavyansk is one of more than a dozen towns and cities pro-Russian gunmen grabbed in east Ukraine since early last month.