There were no reports of bad weather and no obvious reasons why the Boeing 777-200ER would have vanished from radar screens about an hour after it took off from Kuala Lumpur for Beijing. There were no signs of sabotage nor claims of a terrorist attack.
However, in Europe, news reports and officials said at least two people on board may have been carrying stolen passports.
The Italian foreign ministry said an Italian was listed on the flight's manifest, although no national from the country was on board.
The passenger list for Malaysia Airlines flight MH370 provided by the airline included Luigi Maraldi, 37, an Italian citizen. Newspaper Corriere Della Sera reported that Maraldi's passport was stolen in Thailand last August. The Italian interior ministry was unable to immediately comment.
Meanwhile, the Austrian foreign ministry said an Austrian listed among the passengers was safe. He had reported his passport stolen two years ago while in Thailand.
Asked for a possible explanation for the plane's disappearance, Malaysia Airlines chief executive Ahmad Jauhari Yahya told a news conference: "We are not ruling out any possibilities."
However, aviation experts said the two main possibilities look increasingly like catastrophic mechanical failure or an act of terrorism.
Plane maker Boeing said it was monitoring the situation but had no further comment.
This incident comes just a week after 29 people were killed in a mass stabbing in south-western China that authorities blamed on Uighur militants. Security has been stepped up at transport hubs in China. But authorities stressed that no-one has admitted carrying out an attack.
Search operations continued last night, officials said, with planes and ships from several countries scouring the area where the plane last made contact, about halfway between Malaysia and the southern tip of Vietnam.
Vietnam said rescue planes had spotted two large oil slicks and a column of smoke off its coastline, but it was not clear if they were connected to the missing plane. "We sent two maritime boats and some military boats there to clarify," said Pham Quy Tieu, vice-minister of transportation.
Earlier yesterday, the airline had said people from 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.
Flight tracking website flightaware.com showed the Boeing flew north-east over Malaysia after take-off and climbed to 35,000 feet. The flight vanished from the website's tracking records a minute later while it was still climbing.
Chinese relatives of passengers accused the airline of keeping them in the dark. "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 of those on board were taken.
Malaysia Airlines has one of the best safety records of full-service carriers in the Asia-Pacific region. It identified the pilot as Captain Zaharie Ahmad Shah, 53, who joined the airline in 1981 and has 18,365 hours of flight experience.