Comoros has been in a tense stand-off with Anjouan's self-declared leader Mohamed Bacar since a June election which Bacar held in defiance of the federal government.

An African Union (AU) mission, led by South African Foreign Minister Nkosazana Dlamini-Zuma and including senior officials from the United States, France and Tanzania, had been due to arrive last Monday.

But AU and government officials said they did not know when the mission would arrive in the Indian Ocean archipelago.

"We had hoped that a peaceful outcome would be found thanks to the good offices of these countries," Abdourahim Said Bacar, a government spokesman, told Reuters.

"However, we firmly believe dialogue has been exhausted and the military option is the only way to re-establish order on Anjouan and kick out Mohamed Bacar," he said.

His comments came as local news agency, HZK-Presse, reported that preparations were being finalised on another of Comoros' three islands, Moheli, where hundreds of troops have massed in readiness for an attack on Anjouan.

But strong winds and heavy seas were preventing an immediate assault, with a transport ship unable to get into Moheli's port, Inoussa Mohamed, a Moheli resident said.

Comoros' President Ahmed Abdallah Mohamed Sambi has been with his troops since Monday.

On Tuesday, the British foreign office advise against travel to Anjouan and essential travel to the rest of Comoros "due to the increased likelihood of military action".

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

The islands with a population of about 700,000 have endured 19 coups or attempts since independence from France in 1975. First settled by Arab seafarers 1,000 years ago, the islands later became a pirate haven.