MORONI (Reuters) - An assault by the army of Comoros against the rebel island of Anjouan could come at any time, with Ukrainian helicopters in place to reduce risks, the Indian Ocean archipelago's top military officer said on Monday.

Comoros' federal government says military action is the only way to resolve a bitter stand-off with Anjouan, whose self-declared leader held an illegal election eight months ago in order, critics say, to keep himself in power.

"The military assault on Anjouan island is imminent," Lieutenant Colonel Salimou Mohamed Amiri told state radio.

Delivered on Saturday, the two Ukrainian helicopters are intended to cover ground operations, transport soldiers and evacuate the casualties.

"The army could deploy to Anjouan without the helicopters, but that would be very risky," he said.

Hundreds of government troops have assembled on Moheli island, the closest one to Anjouan, in preparation for the likely attack, local media and residents say.

Bad weather in the region had been an obstacle to transporting the troops, Moheli residents said.

But the weather in Comoros' capital, Moroni, was clearer on Monday and the seas much calmer.

Lying off Africa's east coast, the coup-prone Comoros islands retain some autonomy through local leaderships under the terms of a 2001 peace deal, but also share a rotating national president.

First settled by Arab seafarers 1,000 years ago, the islands -- which produce vanilla, cloves and ylang-ylang -- later became a pirate haven.