The Comoros military has made a brief incursion onto the rebel island of Anjouan, capturing three militia members loyal to dissident leader Mohamed Bacar, the Indian archipelago's top military officer said on Wednesday.

Hundreds of national soldiers have amassed on nearby Moheli island for a possible African Union-backed assault against Anjouan to topple Bacar, who is clinging to power in defiance of the archipelago's federal authorities.

"A group of soldiers entered on the 11th in Anjouan's Sima region and arrested three of Mohamed Bacar's militia," Lieutenant Colonel Salimou Mohamed Amiri told Reuters, declining to give more details of the operation.

"This is not the first time that the AND (National Development Army) has made such an intrusion," he said by telephone from Moheli island where the three captured fighters were taken for interrogation.

The Comoros government and the African Union (AU) rejected Bacar's self-organised re-election last year as local president of Anjouan, one of the archipelago's three islands.

Hoping to score a success in Comoros where it has failed in other conflicts like Somalia and Sudan's Darfur region, the AU has pledged to send soldiers to back an attack on Anjouan, a hilly, wooded island that is home to about 300,000 people.

A batch of 500 Tanzanian troops began arriving this week.

Sudan and Senegal are expected to provide another 750 troops, while Libya has offered logistical support for military action that the United States has also said it would support.

With a population of some 700,000 people, the Comoros archipelago has witnessed around 20 coups or attempted coups since independence from France in 1975.

Lying off Africa's east coast, the islands -- which grow ylang-ylang, vanilla and cloves -- were first settled by Arab seafarers 1,000 years ago, then later became a pirate haven.

Anjouan tried to break away from the other islands in 1997, and federal government officials say Bacar, a French-trained former gendarme, wants to do the same again.

He came to power in a putsch in 2001 and has said he wants more autonomy for Anjouan rather than independence.