A Malaysia Airlines flight carrying 227 passengers and 12 crew went missing off the Vietnamese coast on Saturday and was presumed to have crashed.
There were no reports of bad weather and no sign 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 in Rome that an Italian was listed on the flight's manifest although no national from the country was on board, Reuters reported .
The passenger list provided by the airline includes 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 on the report.
In Vienna, the Austrian foreign ministry said an Austrian listed among the passengers was safe and had reported his passport stolen two years ago while he was travelling in Thailand.
Asked for a possible explanation for the plane's disappearance, Malaysia Airlines CEO Ahmad Jauhari Yahya told a news conference: "We are not ruling out any possibilities."
By late on Saturday night, there were no confirmed signs of the plane or any wreckage, over 20 hours after it went missing. Operations will continue through the night, officials said.
Vietnam said its 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, each boat with about 20 people," Pham Quy Tieu, vice minister of transportation, told Reuters by telephone on Saturday evening. "The oil spills are about 15km long. Those boats will be there in about three to four hours."
A crash, if confirmed, would likely mark the U.S.-built airliner's deadliest incident since entering service 19 years ago. And it would also mark the second fatal accident involving a Boeing 777 in less than a year.
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.
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 in Kuala Lumpur. He said 15 air force aircraft, six navy ships and three coast guard vessels had been pressed into service by Malaysia.
Vietnam dispatched two navy boats from Phu Quoc island and sent two jets and one helicopter from Ho Chi Minh City to search for the missing airliner. It was readying a further seven planes and nine boats to join the search effort.
Other than Vietnamese and Malaysian search operations, China and the Philippines have also sent ships to the region to help. The United States, the Philippines, and Singapore also dispatched military planes.
China has also put other ships and aircraft on standby, said Transport Minister Yang Chuantang.
Chinese Foreign Minister Wang Yi told reporters in Beijing that China was "extremely worried" about the fate of the plane and those on board.
Search and rescue vessels from the Malaysian maritime enforcement agency reached the area where the plane last made contact at about 4:30 p.m. local time (4.30 a.m. ET) but saw no sign of wreckage, a Malaysian Maritime Enforcement Agency told Reuters.
The 11-year-old Boeing, powered by Rolls-Royce Trent engines, took off at 12:40 a.m. (12.40 p.m. ET Friday) from Kuala Lumpur International Airport and was apparently flying in good weather conditions when it went missing without a 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.
Malaysia Airlines flight MH370 last had contact with air traffic controllers 120 nautical miles off the east coast of the Malaysian town of Kota Bharu, Malaysia Airlines chief executive Ahmad Jauhari Yahya said in a statement.
Earlier on Saturday, 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 plane flew northeast over Malaysia after takeoff and climbed to an altitude of 35,000 feet. The flight vanished from the website's tracking records a minute later while it was still climbing.
Chinese relatives of passengers angrily accused the airline of keeping them in the dark, while state media criticized the carrier's poor response.
"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."
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, which has still not been 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 carriers in the Asia-Pacific region.
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.