A tourist boat capsized off the coast of India's remote Andaman and Nicobar islands, killing 21 people.

The private boat was carrying 43 tourists, all believed to be Indian nationals, when it sank between the popular spots of Ross Island and North Bay near Port Blair, the capital of the islands.

Thirteen people were pulled from the water following the accident late this afternoon.

They were taken to a hospital in Port Blair, with "several of them seriously injured", an official there said.

Rescue operations were under way with fears for at least another nine people still missing who may be trapped in the now submerged vessel, according to the Press Trust of India.

The Aqua Marine was carrying a large group of tourists from Kanchipuram in India's southern state of Tamil Nadu, the Port Blair official said.

Indian Minister Manmohan Singh expressed shock over the tragedy and "condoled the loss of lives," in a statement, without confirming the number of people killed.

The prime minister said he has asked India's national agencies to help in the rescue and relief operation.

The accident occurred some 25km off the coast of Port Blair, according to the Hindustan Times newspaper.

The Andaman and Nicobar islands, comprising of 572 islands, are located between the Andaman Sea and the Bay of Bengal.

Though they are Indian territory, they are at least 1,000km  from the mainland and are closer to the coast of Myanmar. 

The Department of Foreign Affairs has said it has not received any reports that there were Irish people on board.