A missing AirAsia plane carrying 162 people is presumed to have crashed off the Indonesian coast, an official said on Monday, as countries in the region offered to help Jakarta in the search and recovery effort.

The Indonesia AirAsia plane, an Airbus A320-200, disappeared after its pilot failed to get permission to alter course to avoid bad weather during a flight from the Indonesian city of Surabaya to Singapore on Sunday.

Flight QZ8501 did not issue a distress signal and disappeared over the Java Sea five minutes after requesting a change of course.

"Based on our coordinates, we predict that the plane is on the sea, for now it could be in the bottom of the sea," Soelistyo, head of Indonesia's search and rescue agency, told reporters when asked about the missing plane's likely location.

With Indonesia AirAsia 49 per cent owned by Malaysia-based budget carrier AirAsia, the incident caps a disastrous year for Malaysia-affiliated airlines.

Malaysian Airlines Flight MH370 went missing on March 8 on a trip from Kuala Lumpur to Beijing with 239 passengers and crew and has not been found. On July 17, the same airline's Flight MH17 was shot down over Ukraine, killing all 298 people on board.

Indonesian Air Force spokesman Hadi Thahjanto said two C-130 Hercules planes were focusing the search for Flight QZ8501 in areas northeast of Indonesia's Bangka island, which lies roughly halfway between Surabaya and Singapore, in the Java Sea.

Singapore said it had sent two naval vessels to help while Malaysia said it would send three naval vessels and a C-130. An Australian P3 Orion surveillance plane left Darwin to join the search, the Australian Defence Department said.

The US, Britain, South Korea and India also offered help.

"We have been coordinating with parties from Singapore, Malaysia and Australia who have (expressed) a willingness to assist," Tataog Zainuddin, director of operations at the Indonesian National Search and Rescue Agency, told Reuters.

Onboard Flight QZ8501 were 155 Indonesians, three South Koreans, one person each from Singapore, Malaysia and Britain, while the co-pilot was French.

BAD WEATHER

There was bad weather in the area at the time and the aircraft had been flying at 32,000 feet before asking to fly at 38,000 feet to avoid clouds, said Joko Muryo Atmodjo, air transportation director at Indonesia's transport ministry.

Permission had not yet been given due to traffic in the area, and five minutes later, at 6:17 a.m. on Sunday (2317 GMT Saturday), the plane lost contact with air traffic control, Atmodjo added.

Data from Flightradar24.com, which tracks airline flights in real time, showed several nearby aircraft were at altitudes ranging from 34,000 to 36,000 feet at the time, levels that are not unusual for cruising aircraft.

An A320 pilot from a Southeast Asian airline who has flown the Surabaya-Singapore route many times said it was not unusual to request a slight deviation or climb due to poor weather.

"It is not uncommon at this time of the year, and I have done this myself. It would not have been an unusual request and (air traffic control) will usually grant permission," said the pilot, who declined to be identified because he was not authorised to talk to the media.

The AirAsia group, including affiliates in Thailand, the Philippines and India, has not suffered a crash since its Malaysian budget operations began in 2002. The group's shares in Kuala Lumpur fell as much as 12.9 per cent on Monday.

"We are cooperating with the relevant authorities to the fullest extent to determine the cause of this incident," said Indonesia AirAsia Chief Executive Sunu Widyatmoko.

The Indonesian pilot was experienced and the plane last underwent maintenance in mid-November, the airline said. The aircraft had accumulated about 23,000 flight hours in some 13,600 flights, according to Airbus.

Malaysia AirAsia chief Tony Fernandes flew to Surabaya and, along with Indonesian officials, updated distraught relatives of passengers at a makeshift crisis centre at the airport in Indonesia's second-largest city.

"This is my worst nightmare," Fernandes said on Twitter. "But there's no stopping", he said of the search. – Reuters