Mohamed Bacar, the self-declared president of Anjouan island, has defied the AU and the national authorities in the coup-prone Indian Ocean archipelago since he won an illegal election last June.

Tanzania, Libya, Senegal and Sudan agreed this week to provide military support to help the federal government wrestle back control of Anjouan. It says an assault is now imminent.

Jean Ping, the newly elected AU Commission chief, said Anjouan's leaders had brought the situation upon themselves.

"We would have preferred a solution emerge from the AU," the continent's top diplomat told reporters during a trip to Kenya.

"But Bacar does not want to comply with what has been said by the AU and by the rest of the international community."

The Comoros government's top military officer warned earlier this week that an attack on Anjouan was "imminent", with two Ukrainian helicopters to support the assault. Hundreds of soldiers have gathered on Moheli, the island nearest Anjouan.

Small, hilly Anjouan is home to around 300,000 people. Its local forces look disciplined and well-armed, according to a Reuters reporter who visited recently.

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

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

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