A Malaysia Airlines flight carrying 227 passengers and 12 crew was presumed to have crashed off the Vietnamese coast on Saturday, and European officials said two people on board were using false identities.
There were no reports of bad weather and no sign of why the Boeing 777-200ER would have vanished from radar screens about an hour after it took off from Kuala Lumpur for Beijing.
"We are not ruling out any possibilities," Malaysia Airlines CEO Ahmad Jauhari Yahya told a news conference.
By the early hours of Sunday, there were no confirmed signs of the plane or any wreckage, well over 24 hours after it went missing. Operations will continue through the night, officials said.
There were no indications of sabotage nor claims of a terrorist attack. But the passenger manifest issued by the airline included the names of two Europeans - Austrian Christian Kozel and Italian Luigi Maraldi - who, according to their foreign ministries, were not in fact on the plane.
A foreign ministry spokesman in Vienna said: "Our embassy got the information that there was an Austrian on board. That was the passenger list from Malaysia Airlines. Our system came back with a note that this is a stolen passport."
Austrian police had found the man safe at home. The passport was stolen two years ago while he was travelling in Thailand, the spokesman said.
The foreign ministry in Rome said no Italian was on the plane either, despite the inclusion of Maraldi's name on the list. His mother, Renata Lucchi, told Reuters his passport was lost, presumed stolen, in Thailand in 2013.
U.S. and European security officials said that there was no proof of any terrorist link and there could be other explanations for the use of stolen passports.
The 11-year-old Boeing, powered by Rolls-Royce Trent engines, took off at 12:40 a.m. (1640 GMT Friday) from Kuala Lumpur International Airport and was apparently flying in good weather conditions when it went missing without a distress call.
Flight MH370 last had contact with air traffic controllers 120 nautical miles off the east coast of the Malaysian town of Kota Bharu. Flight tracking website flightaware.com showed it flew northeast after takeoff, climbed to 35,000 feet and was still climbing when it vanished from tracking records.
A crash, if confirmed, would likely mark the 777's second fatal incident in less than a year, and its deadliest since entering service 19 years ago. An Asiana Airlines Boeing 777-200ER crash-landed in San Francisco in July 2013, killing three passengers and injuring more than 180.
Boeing said it was monitoring the situation but had no further comment.
Paul Hayes, director of safety at Flightglobal Ascend aviation consultancy, said the flight would normally have been at a routine stage, having reached initial cruise altitude.
"Such a sudden disappearance would suggest either that something is happening so quickly that there is no opportunity to put out a mayday, in which case a deliberate act is one possibility to consider, or that the crew is busy coping with what whatever has taken place," he told Reuters.
He said it was too early to speculate on the causes.
A large number of planes and ships from several countries were scouring the area where the plane last made contact, about halfway between Malaysia and the southern tip of Vietnam.
"The search and rescue operations will continue as long as necessary," Malaysian Prime Minister Najib Razak told reporters. He said his country had deployed 15 air force aircraft, six navy ships and three coast guard vessels.
Search and rescue vessels from the Malaysian maritime enforcement agency reached the area where the plane last made contact but saw no sign of wreckage, the Malaysian Maritime Enforcement Agency said.
Vietnam said its rescue planes had spotted two large oil slicks, about 15 km (9 miles) long, and a column of smoke off its coastline, but it was not clear if they were connected to the missing plane.
China and the Philippines also sent ships to the region to help, while the United States, the Philippines and Singapore dispatched military planes. China also put other ships and aircraft on standby.
NO DISTRESS CALL
The disappearance of the plane is a chilling echo of an Air France flight that crashed into the South Atlantic on June 1, 2009, killing all 228 people on board. It vanished for hours and wreckage was found only two days later.
John Goglia, a former board member of the National Transportation Safety Board, the U.S. agency that investigates plane crashes, said the lack of a distress call suggested that the plane either experienced an explosive decompression or was destroyed by an explosive device.
"It had to be quick because there was no communication," Goglia said.
He said the false identities of the two passengers strongly suggested the possibility of a bomb.
"That's a big red flag," he said.
If there were passengers on board with stolen passports, it was not clear how they passed through security checks.
International police body Interpol maintains a database of more than 39 million travel documents reported lost or stolen by 166 countries, and says on its website that this enables police, immigration or border control officers to check the validity of a suspect document within seconds. No comment was immediately available from the organization.
Italian police said the passport of Luigi Maraldi was reported stolen on August 1, 2013 and was inserted in the Interpol database
Chinese Foreign Minister Wang Yi told reporters in Beijing that China was "extremely worried" about the fate of the plane and those on board. Chinese passengers' relatives angrily accused the airline of keeping them in the dark, while state media criticized the carrier's response as poor.
"There's no one from the company here, we can't find a single person. They've just shut us in this room and told us to wait," said one middle-aged man at a hotel near Beijing airport where the relatives were taken.
"We want someone to show their face. They haven't even given us the passenger list," he said.
Another relative, trying to evade a throng of reporters, muttered: "They're treating us worse than dogs."
The airline said people of 14 nationalities were among the 227 passengers, including at least 152 Chinese, 38 Malaysians, seven Indonesians, six Australians, five Indians, four French and three Americans.
In Kuala Lumpur, Malaysia Airlines told passengers' next of kin to come to the international airport with their passports to prepare to fly to the crash site, once it was identified.
About 20-30 families were being kept in a holding room at the airport, where they were being guarded by security officials and kept away from reporters.
Malaysia Airlines has one of the best safety records among full-service Asia-Pacific carriers.
It identified the pilot of MH370 as Captain Zaharie Ahmad Shah, a 53-year-old Malaysian who joined the carrier in 1981 and has 18,365 hours of flight experience.