SYDNEY, AUSTRALIA - A boat carrying 40 people has sunk off the Cocos Islands, west of Australia, a report said on Monday.

The Australian Maritime Safety Authority received a distress call on Sunday saying the boat was taking on water about 700 nautical miles northwest of the Cocos Islands, ABC radio's website reported.

A merchant ship from the Bahamas responded to a broadcast for help but found the stricken vessel had sunk when it arrived 12 hours later, and its passengers were in the water.

The merchant ship's crew were trying to rescue passengers and an Australian Defence Force plane was heading to join a search for survivors, ABC said.

The report did not say how many passengers had been accounted for but added that authorities were yet to confirm whether the boat was carrying asylum seekers.

There has been a sharp increase in the number of asylum seekers reaching Australia by boat this year, overloading detention facilities and sparking a heated political battle over border protection and asylum policy.