An AFP journalist said Ukrainian soldiers had taken control of a rebel checkpoint just to the south of the town. Eight armoured vehicles and several soldiers were seen taking position there and warned off anybody trying to approach.
Sporadic small-arms fire was heard at a distance. Two explosions were heard.
A helicopter was seen circling low over the outskirts of Slavyansk, said the rebels, who had set at least one roadblock on fire to produce cloaking smoke.
She said the rebels had shot down a helicopter, but that could not be confirmed.
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 has been warning for several days that the Ukrainian military was preparing an assault.
Kiev is waging an offensive to try to contain the rebels, who continue to expand their grip across east Ukraine.
Slavyansk is one of more than a dozen towns and cities pro-Russian gunmen have grabbed in the region 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.
Moscow denies it has any hand in the east Ukraine unrest, but warns an escalation of the Ukrainian offensive could trigger "catastrophic consequences".
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