Countries around Asia on Tuesday stepped up the search for an AirAsia plane carrying 162 people that is presumed to have crashed in shallow waters off Indonesia, as television showed pictures of objects that could be debris from the jet.

Soelistyo, head of Indonesia's search and rescue agency, told media the search area in the Java Sea between the islands of Sumatra and Borneo would be expanded.

Authorities would also begin scouring smaller islands and coastal land on Indonesian Borneo, while the US said it was sending a warship to help in the search.

There have been no confirmed signs of wreckage from the Airbus A320-200 operated by Indonesia AirAsia, which disappeared in poor weather early on Sunday during a flight from the Indonesian city of Surabaya to Singapore.

The missing plane, with most of those on board Indonesian, could be at the bottom of the sea, Soelistyo said on Monday.

Indonesia's Kompass TV showed pictures of what looked like large, angular objects floating in the sea. One appeared to be orange and another grey or brown. The largest appeared to be several metres long.

"Hopefully, we will find something definite because I haven't received anything else," air force official Dwi Putranto told MetroTV referring to the reported debris.

The Java Sea is relatively shallow, making it easier to spot wreckage in the water, say oceanographers, but strong currents and winds mean any debris would be drifting up to 50 km a day east, away from the impact zone.

"The lesson that should be learned from MH370 is that you need to move quickly," said Charitha Pattiaratchi, an oceanographer at the University of Western Australia, referring to the Malaysia Airlines flight that went missing on March 8 during a trip from Kuala Lumpur to Beijing with 239 passengers and crew and which has not been found.

About 30 ships and 21 aircraft from Indonesia, Australia, Malaysia, Singapore and South Korea would search up to 10,000 sq nautical miles on Tuesday, officials said.

Indonesia's air force spokesman, Hadi Tjahjanto, said authorities would investigate an oil spill seen on Monday, although a separate possible slick turned out to be a reef.

Searchers had investigated several areas where possible debris had been sighted in the water but had found nothing connected to the missing plane, Tjahjanto told Reuters.

Authorities would also investigate reports from fishermen of an explosion on Sunday morning off an island in the area, Tjahjanto added, although dynamite fishing is common in Indonesian waters.

The US military said the USS Sampson, a guided missile destroyer, would be on the scene later on Tuesday.

"We stand ready to assist in any way possible," Pentagon spokesman Mark Wright said.

Flight QZ8501 had sought permission from Indonesian air traffic control to ascend to avoid clouds just before it went missing.

Online discussion among pilots has centred on unconfirmed secondary radar data from Malaysia that suggested the aircraft was climbing at a speed of 353 knots, about 100 knots too slow, and that it might have stalled.

The plane, whose engines were made by CFM International, co-owned by General Electric and Safran of France, lacked real-time engine diagnostics or monitoring, a GE spokesman said. Such systems are mainly used on long-haul flights and can provide clues to airlines and investigators when things go wrong.

Officials said the sea in the general search area was only 50 to 100 metres deep, which should help in finding the plane.

"The Java Sea area where they are now searching isn't even an ocean, it's more of an inland sea," Erik van Sebille, a physical oceanographer at the University of New South Wales in Sydney told Reuters.

"It's so shallow that they may just be able to spot the plane," said van Sebille, noting that sunlight travels through water up to about 100 metres.

Oceanographer Pattiaratchi said debris would normally be expected to float for about 18 days before sinking.

Three airline disasters involving Malaysian-affiliated carriers in less than a year have dented confidence in the country's aviation industry and spooked travellers across the region.

In the third incident, Malaysia Airlines Flight MH17 was shot down over Ukraine on July 17, killing all 298 people on board.

On board Flight QZ8501 were 155 Indonesians, three South Koreans, and one person each from Singapore, Malaysia and Britain. The co-pilot was French.

US law enforcement and security officials said passenger and crew lists were being examined but nothing significant had turned up and the incident was regarded as an unexplained accident.

The plane, which did not issue a distress signal, disappeared after its pilot failed to get permission to fly higher because of heavy air traffic, officials said.

Pilots and aviation experts said thunderstorms, and requests to gain altitude to avoid them, were not unusual in that area.

The Indonesian pilot was experienced and the plane last underwent maintenance in mid-November, the airline said.

Indonesia AirAsia is 49 percent owned by Malaysia-based budget carrier AirAsia.

The AirAsia group, including affiliates in Thailand, the Philippines and India, had not suffered a crash since its Malaysian budget operations began in 2002.

The plane's disappearance comes at a sensitive time for Jakarta's aviation authorities, as they strive to improve the country's safety reputation to match its status as one of the airline industry's fastest growing markets.-Reuters