One official quoted in the Indonesian-language Kompas newspaper on Tuesday said investigators were focusing on the theory that the body of the aircraft was lying deep on the ocean floor off the island of Sulawesi.

Frans Wenas told the paper the few pieces of wreckage found so far showed no signs of burning or explosion, and were spread over a relatively small area, he said.

A mid-air explosion he said, would have dispersed the debris over a large area and more of it would have been found by now.

Any damage sustained subsequently would have been due to the impact with the water as well as undersea pressure, Kompas quoted him as saying.

'Black box'

The US ship USNS Mary Sears
is helping the search [EPA]
Wenas added that search and rescue efforts would become almost impossible once the aircraft’s "black box" flight data recorder stops emitting locator signals, which he said was likely within about 14 days.

A multinational naval search force has not picked up any signal so far.

A US navy vessel will start trawling the waters off Sulawesi soon with equipment designed to pick up the signal in deep ocean, assisted by four sets of detectors sent by Singapore.

Meanwhile the head of Indonesia’s National Transportation Safety Board has told the Associated Press he believes the aircraft probably went into a spiral at cruising altitude before plunging into the sea.

Setyo Rahardjo said other possibilities being looked at included catastrophic structural failure, possibly caused by bad weather.

The aircraft twice changed course after encountering severe weather on its flight before disappearing from radar.

No distress call was heard from the pilots.

As well as officials from the US, Indonesian investigators are being assisted by personnel from Singapore's Aircraft Accident Investigation Bureau, according to Indonesia’s Jawa Pos newspaper.