By Ahmed Ali Amir

MORONI, March 7 (Reuters) - The United States will support any action by the government of Comoros to reunify the Indian Ocean islands, including military force which "now appears necessary", a U.S. envoy said on Friday.

The central government led by President Ahmed Abdallah Mohamed Sambi is preparing an offensive to win back control of Anjouan island whose self-declared leader, Mohamed Bacar, has refused to step down since claiming victory in an illegal election last June.

"Given this implacability, the United States will now support any actions taken by President Sambi, AU (African Union) partners, or the international community to restore Comorian unity," U.S. Ambassador Niels Marquardt said in a statement published by the state-owned weekly newspaper, Al-Watwan.

The envoy, based in Madagascar, said representatives from the AU, Senegal, France, the Arab League and the United States travelled to Comoros last month to present Bacar with a choice: accept free and fair elections or exile.

"Choosing neither option, Colonel Bacar alone must now accept full responsibility for the military conflict that now appears necessary to reunify the Union of the Comoros," Marquardt added.

"His illegitimate hold on power and quest for personal gain will likely and unfortunately lead to bloodshed."

A U.S. embassy official in Madagascar confirmed the comments.

AU troops are due to arrive soon to help Comoros' federal security forces. The AU has led mediation efforts to end the crisis and is keen to secure a rare success to counteract struggling missions in Sudan's Darfur region and Somalia, analysts say.

The federal government of Sambi, a former Islamic preacher, accuses Bacar of secessionist aspirations and fears that Comoros would fall apart if he succeeds in breaking away.

Lying off Africa's east coast with a population of around 700,000, Comoros has a history of coups and inter-island tensions that have discouraged aid and foreign investment.

Washington has no major economic interest in the ex-French colony, which depends mainly on fishing and agriculture. But it wants to foster stability in Comoros, a fragile Muslim nation which produced one of the FBI's most wanted al Qaeda suspects.

The suspect, Fazul Abdullah Mohammed, is still on the run after being accused of involvement in the bombings of U.S. embassies in Kenya and Tanzania in August 1998.

(Writing by Ed Harris; Editing by Katie Nguyen and Mark Trevelyan)

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