An AirAsia plane carrying 162 people that went missing after its pilot failed to gain permission to alter course to avoid a storm is believed to have crashed into the sea, a senior Indonesian rescue agency official said on Monday.

Indonesia was searching the Java Sea on Monday for AirAsia Flight QZ8501, which went missing on Sunday during a flight from the Indonesian city of Surabaya to Singapore.

"Based on our co-ordinates, 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.

"It can be expanded based on evaluation," he told reporters on Monday.

Flight QZ8501 did not issue a distress signal and disappeared five minutes after requesting a change of course on Sunday, said government and transport officials.

Air Force spokesman Hadi Thahjanto said two C-130 Hercules planes were focusing their search efforts 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 look for the Airbus A320-200 operated by Indonesia AirAsia, adding a C-130 took part in the search on Sunday.

Malaysia would send three naval vessels and a C-130 to assist, Singapore's Channel News Asia television reported. An Australian P3 Orion surveillance plane left Darwin to join the search, Australian Broadcasting Corporation radio said. The United States, Britain, South Korea and India also offered help.

"We have been co-ordinating 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.

Malaysia-based budget carrier

Indonesia AirAsia is 49 per cent owned by Malaysia-based budget carrier Air Asia. The AirAsia group, including affiliates in Thailand, the Philippines and India, has not suffered a crash since its Malaysian budget operations began in 2002.

Relatives of the passengers of AirAsia flight QZ8501 comfort each other at Juanda International Airport in Surabaya, Indonesia. (Trisnadi/Associated Press) Onboard Flight QZ8501 were 155 Indonesians, three South Koreans, one person each from Singapore, Malaysia and Britain, while the co-pilot was French. Canada's Foreign Affairs Department said there's no indication there were any Canadians on board, but officials are working to confirm that with local authorities.

There was bad weather in the area at the time and the aircraft had been flying at 9,700 metres before asking to fly at 11,600 metres 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, the plane lost contact with air traffic control, Atmodjo added.

The pilot "was requesting deviation due to en-route weather before communication with the aircraft was lost," the airline said.

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

'Worst nightmare'

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.

Family members of passengers on AirAsia Flight QZ8501 search a list of names as they await news of their loves ones at Juanda Airport in Indonesia. (Suryanto/Antara Foto/Reuters) 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.

AirAsia swapped the distinctive bright red of its logo for grey on social media accounts and the company website as it faced its biggest ever challenge.

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 on board 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.

Praying for passengers

Indonesian President Joko Widodo urged his people to pray for the safety of the passengers and crew. During his Sunday address at St. Peter's Square in the Vatican, Pope Francis said those on board were in his prayers.

Louise Sidharta was at Singapore's Changi Airport waiting for her fiancée to return from a family holiday.

"It was supposed to be their last vacation before we got married," she said.

A man named Purnomo told TVOne in Surabaya of his lucky escape. "I should have been on the flight ... but this morning I had an emergency. I had my passport in hand."

Like all affiliates of AirAsia, Indonesia AirAsia operates Airbus jets, of which it has 30 of the A320 model.

A Changi Airport employee in Singapore holds up a sign to direct possible relatives of passengers of AirAsia Flight QZ8501. (Edgar Su/Reuters) AirAsia has ordered several hundred jets from the European plane-maker, making it one of its most important customers. The missing plane has been in service for just over six years, according to airfleets.net.

Indonesian officials from the civil aviation authority and transport safety committee, which are responsible for crash investigations, arrived in Surabaya on Sunday. A Transport ministry official said Indonesia would handle the probe.

"The aircraft was registered in Indonesia and it looks to be missing over Indonesian territory, so we will lead the investigation," said the official, who asked not to be named as he was not authorized to speak to the media.

"We have the expertise to do this."