The search continues for AirAsia Indonesia flight QZ8501, a day after it went missing with 162 people on board.
"Based on our coordinate estimation, initial estimation is [the plane is] in the water," said Bambang Soelistyo, head of Indonesia's search and rescue agency, speaking in Jakarta.
However, no evidence has so far been found of the plane's whereabouts.
Officials in Surabaya, from where the plane took off, said search teams were only just reaching the area.
The Airbus A320-200 disappeared over the Java Sea early on Sunday on a flight to Singapore.
The pilots had requested a course change due to bad weather but did not send any distress call before the plane disappeared from radar screens.
"Based on the coordinates given to us and evaluation that the estimated crash position is in the sea, the hypothesis is the plane is at the bottom of the sea," Mr Soelistyo told a press conference.
"That's the preliminary suspicion and it can develop based on the evaluation of the result of our search," he said.
That search was largely suspended as night fell on Sunday.
Although some ships continued the hunt overnight, the main search planes and vessels, from several nations, only resumed at first light on Monday.
Mr Soelistyo said Indonesia was providing 12 ships, three helicopters and five military aircraft.
Indonesian transport ministry official Tatang Kurniadi said: "Our primary task is to find the plane. We are co-operating with every possible department, and relevant countries.
"This includes the department responsible for aircraft design, the aircraft manufacturer from France, and relevant operational teams from AirAsia and Malaysia."
Malaysia was to deploy a C-130 plane, along with three ships, with Singapore lending a C-130 and Australia also providing help.
The search area is near Belitung island.
AirAsia's share price fell 7% in morning trading on Monday in Kuala Lumpur.
Flight QZ8501 had left Surabaya in eastern Java at 05:35 local time on Sunday (22:35 GMT Saturday) and was due to arrive in Singapore at 08:30 (00:30 GMT).
Last contact was at 06:24 local time.
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.
Indonesian officials said the request could not be immediately approved due to traffic, but the plane disappeared from the radar screens before the pilots gave any further response.
AirAsia boss Tony Fernandes said this was his "worst nightmare".
Mr Fernandes flew to Surabaya and later said: "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."
Rescuers check search coordinates in Surabaya
Oceanographer Simon Boxall told the BBC the plane should not be too difficult to find if it went into the water.
The sea floor is within diver depth, he says, and it would be "likely that they'll get answers within a few days".
Difficult yearThe AirAsia Indonesia plane in question was delivered in 2008, has flown 13,600 times, completing 23,000 hours, and underwent its last maintenance on 16 November.
The captain, Iriyanto, had 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.
The AirAsia group has previously had no fatal accidents involving its aircraft.
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 Juanda international airport in Surabaya.
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.
What is the AirAsia Group?
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