"We must wait until the National Commission for Transportation Safety has located the ill-fated plane."

Indonesian civilian and military rescuers have been hunting for the Boeing 737-400 over dense jungle and rugged mountains in western Sulawesi while navy vessels have been combing the Makassar Strait between Sulawesi and Borneo.

Eddy Suyanto, commander of Hasanudin air base, said the Singaporean air force's two jets would join two helicopters and six aeroplanes on the Indonesian side to try to locate the missing Adam Air airliner.

Heavy rains, strong winds and rough terrain continued to hamper rescue operations on Thursday.

Separately, 15 survivors of a ferry sinking last week were picked up by a fishing vessel on Wednesday, taking the number of survivors to 230 out of the more than 600 on board.

About 400 people are still missing.

The Nusantara Senopati ferry went down in the Java sea just before midnight on Friday during a violent storm.

Ruth Simatupang, a government investigator, blamed four metre-high waves which flooded the lower deck of the ferry, causing it to sink.

"I suspect waves entered the car deck over the door and became trapped, making the vessel too heavy and unstable," she said.

She said the vessel was not overcrowded because "everyone could sit down, sleep and there was space on the car deck".

A man identified as Wiranto, believed to be the captain of the Nusantara Senopati, is being investigated, Lieutenant-Colonel Tony Syaiful, a navy spokesman, said.