"When the military preparations are finished, nothing can stop this operation to restore constitutional order," national government spokesman Abdourahim Said Bacar told Reuters.

Anjouan's self-declared leader, Mohamed Bacar, has defied the national government of the Indian Ocean archipelago since an illegal election in June that he claimed to win.

On Tuesday, the AU extended a travel ban on 145 Anjouanais officials including Bacar for two months, and other punitive measures imposed in October such as a freezing of funds and other financial assets.

Comoros has been waiting for an AU mediation mission since early last week, but no date for the trip has been announced and the government says its patience is running out.

"We cannot give two more months to Colonel Mohamed Bacar by applying the same sanctions which have had no effect so far," the government spokesman said.

The federal government's top military officer said on Monday an attack was imminent, with two Ukrainian helicopters to support the assault. Hundreds of troops have gathered on Moheli, the island closest to Anjouan.

Previous attempts by the federal government in 1997 and 2007 to wrestle control of the island both failed.

Home to about 300,000 people, Anjouan is a small, hilly, and wooded island. Its local forces seem disciplined and well-armed, according to a Reuters reporter who visited recently.

Lying off Africa's east coast, the Comoros is a fragile state with a population of about 700,000, which is trying to shake off a history of coups and inter-island friction.

First settled by Arab seafarers 1,000 years ago, the tropical islands later became a pirate haven. They have been independent from France since 1975.