Tensions are rising in Comoros, where one of the three main islands has installed a new leader in defiance of the national government.

Mohamed Bacar was sworn in as president of Anjouan Thursday, a day after the government said it was considering unspecified military action.

The government has the backing of the African Union, whose envoy to Comoros, Francisco Madeira said Thursday that all options are open to deal with the situation.

Bacar took office on the basis of an election last Sunday that Comoros officials have declared null and void. The national government had ordered Anjouan to delay the election for a week because of security concerns.

Last month, two people were killed when forces loyal to Bacar clashed with the national army.

Last week, gunfire at the island's airport prevented the plane of Comoros' national president from landing.

Bacar served a previous term as elected president of Anjouan after seizing power on the island in a 2001 coup.

The Union of Comoros consists of three islands in the Indian Ocean - Grand Comore, Anjouan and Moheli. Each has its own president and shares a rotating national presidency.

The country has suffered 19 coups or coup attempts since winning independence from France in 1975. Comoros' current political setup was established in 2001 after Anjouan and Moheli threatened to secede.

Some information for this report was provided by AFP and Reuters.