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 spokeswoman 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.
The Ukrainian government and the West believe the insurgency is being fomented by Moscow, which in March used the same tactics ahead of its annexation of Crimea.
RESULT
Slavyansk (Ukraine) (AFP) - Gunfire and heavy detonations were heard Friday just to the north of Ukraine's rebel-held town of Slavyansk, insurgents and AFP
Gunfire, Heavy Detonations Heard Near Ukraine Rebel Town ...
Slavyansk, Ukraine: Gunfire and heavy detonations were heard on Friday just outside Ukraine's rebel-held town of Slavyansk, in what the insurgents said was
Gunfire, heavy detonations heard near Ukraine rebel town ...
SLAVYANSK, Ukraine - Gunfire and heavy detonations were heard Friday just outside Ukraine's rebel-held town of Slavyansk, in what the insurgents said was "a full
Gunfire, heavy detonations heard near Ukraine rebel town ...
'Gunfire, heavy detonations heard near Ukraine rebel town' on Yahoo News UK. Gunfire and heavy detonations were heard Friday just to the north of Ukraine's rebel-held
Gunfire, heavy detonations heard near Ukraine rebel town ...
From Yahoo News Singapore: Gunfire and heavy detonations were heard Friday just to the north of Ukraine's rebel-held town of Slavyansk, insurgents and AFP journalists
Gunfire heard near Ukraine rebel town
SLAVYANSK: Gunfire and heavy detonations were heard Friday just outside Ukraine's rebel-held town of Slavyansk, in what the insurgents said was "a full-scale attack
Gunfire, heavy detonations heard near Ukraine rebel town ...
Read 'Gunfire, heavy detonations heard near Ukraine rebel town' on Yahoo News South Africa. Gunfire and heavy detonations were heard Friday just to the north of
Gunfire, heavy detonations heard near Ukraine rebel town ...
Gunfire and heavy detonations were heard Friday just outside Ukraine's rebel-held town of Slavyansk, in what the insurgents said was "a full-scale attack
Gunfire, heavy detonations heard near Ukraine rebel town ...
Gunfire, heavy detonations heard near Ukraine rebel town; In US heartland, Tea Party tests GOP establishment; 58 arrested in Philippines over global
Digital Journal: A Global Digital Media Network
heavy-detonations-heard-near-ukraine-rebel-town/article/382684: Gunfire, heavy detonations heard near Ukraine Ukraine's rebel-held town
