Officials say two passengers on the missing Malaysia Airlines jet were traveling with stolen passports.
Two men listed on the flight's manifesto — one from Italy and another from Austria — never boarded the flight from Kuala Lumpur to Beijing that disappeared early Saturday carrying 239 people. Both men had their passports stolen in Thailand in the last two years. It is not clear who was flying with the stolen documents.
The Austrian is in his home country and the Italian is still living in Thailand.
U.S. officials say they are still looking at the disappearance as if it is an accident, but Malaysian officials say they are not ruling anything out.
The Vietnamese government sent rescue boats where search planes spotted two large oil slicks off the southern tip of the country shortly after the airliner vanished on Saturday.
The slicks — each about 15 kilometers long — are the first potential traces found since Malaysia Airlines Flight MH370 from Kuala Lumpur to Beijing disappeared early with 239 passengers and crew on board.
The Pentagon has dispatched a naval destroyer and a surveillance plane to aid in the search, and ships and aircraft from Malaysia, Vietnam, China and the Philippines have concentrated their search in an area about 240 kilometers off the coast of Vietnam's southwestern Tho Chu island. Vietnamese authorities say that is where they last detected a signal from the Boeing 777-200.
"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.
A Vietnamese naval commander had told state media that the missing plane could have crashed in Malaysian waters.
However, Malaysian acting Transport Minister Hishamuddin Tun Hussein told a news conference that he had not been informed that the plane had been located and no wreckage has been sighted.
"We are doing everything in our power to locate the plane. We are doing everything we can to ensure every possible angle has been addressed," Transport Minister Hishamuddin Hussein told reporters near the Kuala Lumpur International Airport.
"We are looking for accurate information from the Malaysian military. They are waiting for information from the Vietnamese side," he said.
Vanished after reaching 35,000 feet
The airline said it lost all contact with Flight MH370 about an hour after it took off from the Malaysian capital early Saturday morning local time.
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.
At a news conference Saturday, he said the airline was working with search and rescue teams to locate the aircraft and was calling the families of the passengers and crew.
The company's Facebook page said people from 14 nationalities were among the 227 passengers, including at least at least 152 Chinese, 38 Malaysians, seven Indonesians, six Australians, five Indians and four French. State Department spokeswoman Jen Psaki said three Americans were on board the flight.
"The Australian government fears the worst for those aboard missing Malaysia Airlines flight MH370," a spokeswoman for Australia's Department of Foreign Affairs and Trade said.
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.
China's official Xinhua news agency said contact with the plane was lost in Vietnamese airspace. It said the plane never entered China's air traffic control area. Vietnamese officials said the flight disappeared about a minute short of entering Vietnamese airspace.
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 relatives of passengers angrily accused the airline of keeping them in the dark, while state media criticised 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.
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.
The flight left Kuala Lumpur around 12:40 a.m. (1640 GMT Friday) and was due to land in the Chinese capital at 6:30 a.m. (2230 GMT Friday) the same day.
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.
Chinese state media said 24 Chinese artists and family members, who were in Kuala Lumpur for an art exchange programme, were aboard. The Sichuan provincial government said Zhang Jinquan, a well-known calligrapher, was on the flight.
If it is confirmed that the plane crashed, the loss would mark the second fatal accident involving a Boeing 777 in less than a year and by far the worst since the jet entered service in 1995.
The most recent accident involving a Boeing 777 was the Asiana Airlines crash at the San Francisco International Airport in July, 2013, in which three people died. Pilot error is suspected in that incident.
Boeing said it was monitoring the situation but had no further comment. The flight was operating as a China Southern Airlines codeshare.
Some information for this report was provided by AP and AFP and Reuters.