Western Australia Police Commissioner, Karl O'Callaghan

Rescue efforts are under way after a boat believed to be carrying about 200 asylum-seekers capsized off Christmas Island, Australian officials say.

The Australia Maritime Safety Authority (AMSA) said the boat sank in Indonesian waters, 120 nautical miles (220km) north of the island, reports the BBC.

At least 40 people have been seen on the hull of the vessel, but police said many others were feared to have died.

Indonesian and Australian rescue vessels were in or heading to the area.

"There were about 200 refugees on board we think. Currently there's about 40 on the hull and the rest are in the water," said Western Australia Police Commissioner, Karl O'Callaghan.

"Some of the very early reports suggest that up to 75 people may have drowned, but I do stress that they are unconfirmed at this stage," he said.

Christmas Island lies off Australia's north-west coast. It is closer to Indonesia than Australia, and is targeted by asylum-seekers hoping to get to Australia.

These boats are often over-loaded and poorly maintained. About 50 asylum-seekers died when their boat broke up on rocks off Christmas Island in December 2010.

Jo Meehan, a spokesperson for AMSA, said that an Australian customs surveillance aircraft alerted had them to the capsized boat.

She said three merchant vessels and two Australian patrol boats were in or near the area and that these had night searching capabilities that could aid in the rescue.