A missing AirAsia jet carrying 162 people could be at the bottom of the sea after it was presumed to have crashed off the Indonesian coast, an official has said, as countries around Asia sent ships and planes to help in the search.

The Indonesia AirAsia plane, an Airbus A320-200, disappeared after its pilot failed to get permission to fly higher 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 the change of course, which was refused because of heavy air traffic.

"Based on our coordinates, we expect it is in the sea, so for now (we think) it is on the sea floor," the head of Indonesia's search and rescue agency, told reporters when asked about the missing plane's likely location.

Transport Minister Ignasius Jonan said the search was focused on an area 70 nautical miles square between the island of Belitung, off Sumatra, and Borneo island.

He said the sea was only 50 to 100 metres deep in the area, which would be a help in finding the plane. Ships were hunting around the clock but aircraft would suspend their operations at dusk, he added.

Air force spokesman Hadi Tjahjanto said searchers were checking a report of an oil slick off Belitung. Searchers had picked up an emergency locator signal off the south of Borneo but no subsequent signal was found, he said.

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

The disappearance caps a disastrous year for Malaysia-affiliated airlines, with Indonesia AirAsia 49 percent owned by Malaysia-based budget carrier AirAsia.

Malaysia 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.

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 group's shares in Kuala Lumpur closed 8.5 percent lower.

ANGER GROWING

Indonesian Vice President Jusuf Kalla, visiting relatives of people on the flight in Surabaya, told reporters the search by 30 ships and 15 aircraft, about halfway between Surabaya and Singapore, was being hampered by bad weather.

Anger was growing among about 100 relatives at a crisis centre at the airport in Indonesia's second-largest city.

"We only need clear information every hour on where they are going," said Franky Chandra, who has a sibling and three friends on the flight, referring to the search teams.

"We've been here for two days but the information is unclear. That's all we need."

Australia, Malaysia, Singapore and South Korea have sent ships and aircraft to join the search, and China offered to send planes and ships and any other help Indonesia needs.

Soelistyo said Indonesia might not have the best technology to search underwater and had accepted offers of help from the United States, Britain and France.

In 2007, it took Indonesia months to recover flight data recorders from a Boeing 737-400 operated by Indonesia's Adam Air which crashed off Sulawesi island, killing all 102 on board.

Flight QZ8501 was travelling at 32,000 feet (9,753 metres) and had asked to fly at 38,000 feet to avoid clouds.

Permission was not given at the time due to traffic in the area. Five minutes later, at 6.17 a.m. on Sunday (2317 GMT Saturday), the plane lost contact with air traffic control, transport officials said.

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.

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

"The airplane's performance is directly related to the temperature outside and increasing altitude can lead to freezing of the static radar, giving pilots an erroneous radar reading," said a Qantas Airways pilot with 25 years' experience flying in the region.

The resulting danger is that pilots take incorrect action to control the aircraft, said the pilot, who requested anonymity.

Online discussions among pilots centred on unconfirmed secondary radar data from Malaysia that suggested the missing plane was climbing at a speed of 353 knots, about 100 knots too slow in such weather conditions.

"At that altitude, that speed is exceedingly dangerous," Sydney-based aviation expert Geoff Thomas said.

"At that altitude, the thin air, the wings won't support the aircraft at that speed and you get an aerodynamic stall."

Safety authorities say accidents involving a loss of control, such as might occur in bad weather, are rare but almost always catastrophic.

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