SYDNEY - ABOUT 15 people were feared dead on Monday after a boat carrying 39 sank in rough seas far off north-west Australia despite frantic rescue attempts by a passing merchant ship and fishing vessel.

Defence chief Angus Houston said the unidentified boat capsized and then sank after the LNG Pioneer ship and Taiwanese fishing craft responded to pleas for help in a remote area off Australia's Cocos Islands in the Indian Ocean.

It was not confirmed whether the boat was one of dozens of people-smuggling vessels that have headed to Australia this year carrying more than 1,700 asylum-seekers, many of them from Sri Lanka and Afghanistan.

Some 19 people were rescued by the Bahamas-based gas tanker and fishing boat, while an air force plane dropped a life raft near another five survivors, Prime Minister Kevin Rudd said.

'I think it's fair to say there are grave concerns about a number of those people - we understand there were something like 39 people on board this vessel,' Mr Rudd told Fairfax radio.

The Australian Customs Service said rescue aircraft returned to base on Monday night but would resume the search for survivors on Tuesday, according to the AAP news agency. -- AFP