* Rebels say operation begins, fighting in northeast

* Say shot down military helicopter, take pilot prisoner

* Kiev says no comment until it's over (Adds more detail, background)

By Maria Tsvetkova

SLAVIANSK, Ukraine, May 2 (Reuters) - Pro-Russian separatists in the rebel stronghold of Slaviansk said on Friday Ukrainian forces had launched a "large-scale operation" to retake the eastern town, with fighting on its northeastern outskirts.

The rebels in Slaviansk, the eastern Ukrainian town where the armed separatists are most firmly in control, said at least one military 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.

Ukrainian officials, who have been criticised for being slow to act to stop the rebels seizing swathes of its Donbass coal and steel belt, launched an "anti-terrorist" operation in early April but have resisted large-scale measures so far.

In Kiev, an aide to Interior Minister Arsen Avakov said he could not comment. "Until it's over, no one will say anything," he said.

The rebels, who demand a referendum on independence for at least two eastern regions on May 11, have seized official buildings in major centres and small towns across the Donbass, often with little or no opposition from local police.

Kiev has acknowledged it has little control over some of the police forces in the east, threatening the status of a presidential election due on May 25 which some in Kiev hope will draw a line under the messy transitional period.

A new government was installed in Kiev in February after President Viktor Yanukovich fled the capital. (Additional reporting by Natalia Zinets, Writing by Elizabeth Piper; Editing by Timothy Heritage)

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