Rescuers pulled six people from the sea off Indonesia but authorities fear for over 140 more after a suspected refugee boat disappeared on its way to Australia.
Indonesian authorities are coordinating the search for the boat, which first reported to be in distress yesterday in Indonesia's Sunda Strait.
Several merchant ships and rescue helicopters joined the operation, around 42 nautical miles off Java.
Australia's Home Affairs Minister Jason Clare said authorities believe the boat was carrying 150 people.
Refugees seeking asylum in Australia often set sail from Indonesia heading for Australia's Indian Ocean territory of Christmas Island in dangerous and overcrowded boats, with the help of people smugglers.
Since 2001, almost 1,000 people have died at sea while attempting to reach Australia on overcrowded and often unseaworthy refugee boats from Indonesia.
A person on board the unidentified boat had contacted Australian rescue authorities by telephone yesterday, saying the vessel had engine trouble and needed assistance.
A search by Indonesian authorities failed to locate the boat, but a merchant vessel early today reported it had seen people in the water and was attempting to recover survivors, Australia's Maritime Safety Authority said.
The UN refugee agency said Australia received 11,800 claims for asylum in 2011, compared with 441,000 claims globally, with 327,000 of those claims in Europe.
Australia is working on plans to reopen refugee detention centres in the Pacific islands nations of Nauru and Papua New Guinea to deter refugee boats from reaching Australia and prevent sinkings.