AirAsia boss Tony Fernandes says he is "devastated" that a plane carrying 162 people from Indonesia to Singapore has gone missing.
AirAsia Indonesia flight QZ8501, an Airbus A320-200, disappeared over the Java Sea an hour into its flight at 06:24 local time (23:24 GMT Saturday).
Bad weather was reported in the area but no distress call was made and no wreckage has been sighted.
The search operation was halted for the night and resumes at about 23:00 GMT.
The plane had left Surabaya in eastern Java at 05:35 local time (22:35 GMT Saturday) and was due to arrive in Singapore at 08:30 (00:30 GMT).
Shocked relatives gathered at both airports, in Singapore and Surabaya, poring over the passenger manifests.
'My worst nightmare'
Mr Fernandes, AirAsia's chief executive, flew to Surabaya and later told a press briefing: "We are very devastated by what's happened, it's unbelievable.
"Our concern right now is for the relatives and for the next of kin - there is nothing more important to us, for our crew's family, and for the passengers' families.
"We hope that the aircraft is found quickly, and we can find out the cause of what has happened."
An air and sea search was immediately launched, with vessels and aircraft from Indonesia, Singapore and Malaysia scouring an area near Belitung island, but nothing was found before nightfall.
The Indonesian search and rescue force will resume at dawn with 12 ships, three helicopters and five aircraft. There will also be two C-130 planes - one each from Malaysia and Singapore - and three ships from those nations.
Dense storm clouds were reported in the area at the time the plane lost contact.
The pilot had asked permission to climb to 38,000ft (11,000m) to avoid the clouds.
But Indonesian transport ministry official Djoko Murjatmodjo said the request "could not be approved at that time due to traffic, there was a flight above, and five minutes later [flight QZ8501] disappeared from radar".
Geoffrey Thomas, of airlineratings.com, told the BBC that radar plots had shown the plane was flying at 353 knots, 100 knots slower than it should have been.
Although he stressed it was only speculation, he said it was possible the pilots may have lost data on air speed because of an ice-up of pitot tube instrumentation. This was thought to have had happened in the loss of the Air France Airbus A330 over the Atlantic Ocean in 2009.
A recent safety study showed only 10% of fatal incidents occurred at cruising altitude.
The AirAsia group has previously had no fatal accidents involving its aircraft.
Mr Fernandes, who has owned AirAsia since 2001 and turned it into a highly successful operation, tweeted: "This is my worst nightmare."
The AirAsia Indonesia plane in question was delivered in 2008, has flown 13,600 times, covering 23,000 hours, and underwent its last maintenance on 16 November.
The captain, Iriyanto, has more than 20,500 flight hours, almost 7,000 of them with AirAsia, Mr Fernandes said. The co-pilot is French national Remi Emmanuel Plesel.
AirAsia has set up an emergency line for family or friends of those who may be on board. The number is +622 129 850 801.
Special centres were set up at both Singapore's Changi airport and the Juanda international airport in Surabaya.
AirAsia Indonesia CEO Sunu Widyatmoko said the company had "mobilised a support team to help take care of [relatives'] immediate needs, including accommodation and travel arrangements".
There were 155 passengers on board, the company said in a statement:
- 137 adults, 17 children and one infant
- Most were Indonesian but several were from other countries: one UK national, a Malaysian, a Singaporean and three South Koreans
- The BBC understands that the British national is Chi-Man Choi
- Two pilots and five crew were also on board - one French, the others Indonesian
This has been a difficult year for aviation in Asia: Malaysia's national carrier Malaysia Airlines has suffered two losses - flights MH370 and MH17.
Flight MH370 disappeared on a flight from Kuala Lumpur to Beijing in March with 239 passengers and crew. The wreckage, thought to be in southern Indian Ocean, has still not been located.
MH17 was shot down over Ukraine in July, killing all 298 on board.
- Low-cost airline group with main hub in Kuala Lumpur, Malaysia. Founded in 2001
- AirAsia Group services now comprises eight affiliates, including AirAsia Indonesia
- Group employs more than 15,000 staff
- Operates fleet of more than 150 Airbus A320 aircraft with 200 more on order
- Flies to about 100 destinations in some 15 countries. Carried almost 8 million passengers in 2013
- Tony Fernandes has been the group's CEO since December 2001
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