China has demanded that Malaysia turn over satellite data it used to conclude that a passenger jet was lost in the southern Indian Ocean with no survivors during a flight to Beijing.
Among the Malaysia Airlines flight's 239 passengers, 153 were Chinese nationals, making the incident a highly emotional one for Beijing.
Relatives of the missing passengers have complained bitterly about a lack of reliable information and some suspect they are not being told the whole truth.
Deputy foreign minister Xie Hangsheng told Malaysia's ambassador to Beijing that China wanted to know what exactly led Malaysia to announce that the plane had been lost, China's Foreign Ministry said on its website.
"We demand the Malaysian side to make clear the specific basis on which they come to this judgment," Mr Xie was quoted as telling Datuk Iskandar Bin Sarudin during their meeting.
There was no immediate response from the Malaysian side.
The announcement sparked mournful, angry and chaotic scenes at the Beijing hotel where relatives had gathered.
A group of family members read out a statement condemning Malaysia Airlines and the Malaysian government and military and vowing to hold them responsible for the deaths of their loved ones.
Relatives plan to stage a further protest outside the Malaysian embassy in Beijing.
The plane vanished less than an hour into an overnight flight on March 8 from Kuala Lumpur to Beijing.
After 17 days of desperation and doubt over the missing jet, Malaysia said an analysis of satellite data pointed to a "heartbreaking" conclusion: Flight 370 met its end in the southern reaches of the Indian Ocean, and none of those aboard survived.
The sombre announcement by prime minister Najib Razak left unresolved many more troubling questions about what went wrong aboard the Boeing 777 to take it so far off-course.
Mr Najib read a brief statement about what he called an unparalleled study of the jet's last-known signals to a satellite. That analysis showed that the missing plane veered "to a remote location, far from any possible landing sites".
"It is therefore with deep sadness and regret that I must inform you that, according to this new data, Flight MH370 ended in the southern Indian Ocean," he said.
His carefully chosen words did not directly address the fate of those aboard. But in a separate message, sent to some of their relatives just before he spoke, Malaysia Airlines said "we have to assume beyond any reasonable doubt that MH370 has been lost and that none of those on board survived".
Officials said they concluded that the flight had been lost in the deep waters west of Perth, Australia, based on more thorough analysis of the brief signals the plane sent every hour to a satellite belonging to Inmarsat, a British company, even after other communication systems on the jet shut down.
The pings did not include any location information. But Inmarsat and British aviation officials used "a type of analysis never before used in an investigation of this sort" to zero in on the plane's last direction, as it reached the end of its fuel, Mr Najib said.
Inmarsat said the company used "detailed analysis and modelling" of transmissions from the Malaysia Airlines jet and other known flights to describe "the likely direction of flight of MH370".
Mr Najib gave no indication of exactly where in the Indian Ocean the plane was last heard from, but searchers have sighted possible debris in an area about 1,550 miles south west of Perth.
High waves, gale-force winds and low-hanging clouds forced the multinational search to be suspended for 24 hours, the Australian Maritime Safety Authority said.
Australian Transport Minister Warren Truss, responsible for the search co-ordination, said the determination that the plane had crashed shifts the search to a new phase, but that it would be a difficult and long one.
"The Malaysian announcement is purely based on the satellite imagery that's available, the calculations about fuel and capacity of the aircraft to stay in the air, so it's really a long, long way away before much can be done by way of physical examination," he said.
He said the Australian naval supply ship HMAS Success had been in the area where objects had been spotted Monday, but its crew had been unable to find anything.
There is also a race against the clock to find any trace of the plane that could lead them to the location of the black boxes, the common name for the cockpit voice and data recorders, whose battery-powered "pinger" could stop sending signals within two weeks. The batteries are designed to last at least a month and can last longer.
The US Pacific Command is sending a black box locator in case a debris field is located.
The Towed Pinger Locator, which is pulled behind a vessel at slow speeds, has highly sensitive listening capability that can hear the black box pinger down to a depth of about 20,000 feet.
The US Navy has also sent an unmanned underwater vehicle to Perth that can be used if debris is located.
Meanwhile, the Australian Maritime Safety Authority (AMSA) said gale force winds, heavy rain and low cloud meant planes could not fly safely, and heavy seas meant an Australian navy ship was leaving the area where possible debris had been sighted yesterday.