However, Vietnamese authorities said they had spotted a 14-mile long oil slick 120 miles off the coast of Cape Ca Mau - the most southerly point of Vietnam's mainland.
A Vietnamese government statement said the slicks were spotted late on Saturday off the southern tip of Vietnam and were each between six and nine miles long. There was no confirmation that the slicks were related to the missing plane, but the statement said they were consistent with the kinds that would be produced by the two fuel tanks of a crashed jetliner.
Ahmad Jauhari Yahya, the chief executive of Malaysia Airlines, said there was no indication that the pilots had sent a distress signal, suggesting that whatever happened to the plane occurred quickly and possibly catastrophically.
"We have no information on the location of the aircraft," said Malaysia Airlines in a statement. "We are currently working with international authorities on the search and rescue mission."
The search has focused on an area of the South China Sea roughly 120 nautical miles south west of Vietnam - the last point of contact with the jet, and where the oil slick has been reported.
China, Vietnam, Malaysia and the Philippines all dispatched rescue ships and an emergency rescue message also alerted all ships in the region to assist the mission and watch for any survivors that might be adrift.
The Pentagon dispatched a naval destroyer and a surveillance plane to aid in the search. The US military statement said a P-3C Orion reconnaissance plane also will depart shortly from Kadena Air Base in Okinawa, Japan
The flight, a codeshare with China Southern airlines, had 153 Chinese on board, including one child. There were also five Indians, four French, three US citizens, two passengers each from New Zealand, Ukraine, and Canada, and one each from Russia, Italy, the Netherlands and Austria, but no British passengers.
Among the Chinese were two groups, one of artists and their families, who had taken part in a cultural exchange and another of Buddhists returning from a religious meeting in the Malaysian capital.
Relatives of the passengers were directed by the Chinese police to the Lido Hotel in Beijing where they waited for news in a large conference room.
Angry relatives accused the airline of keeping them in the dark and failing to provide updates as they were received.
About 20 people stormed out of the room at one point, enraged they had been given no information.
"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, who declined to give his name.
"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."
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.
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."
Hishamuddin Hussein, Malaysian transport minister, said it was too early to confirm a crash and that there were no signs of wreckage on the surface of the water.
"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," he said.
Flight MH370, a Boeing 777-200 jet, took off from Kuala Lumpur at 12.35am on Saturday. Two hours later, according to Malaysia Airlines, it lost contact with the air traffic control in Vietnam.
The last data from the plane showed it climbing steadily to 35,000 ft before vanishing. There was no distress signal and the weather conditions were good.
Vietnam authorities said contact with Flight MH370 was lost near its airspace, but its exact location and fate remained a mystery for more than 12 hours after it slipped off air-traffic control screens.
In Beijing, the Chinese president Xi Jinping ordered all resources to be mobilised to find the plane and aid survivors.
"We are extremely worried," said Wang Yi, the Chinese Foreign minister, breaking off a press conference to attend to the crisis.
Frustrated relative struggled to make sense of the disappearance of the Boeing 777-200 which - like the Malaysian national carrier - has a solid safety record.
Malaysia Airlines said the plane, on an overnight flight from Kuala Lumpur to Beijing, relayed no distress signal, indications of rough weather, or other signs of trouble.
"We filled in a form with our personal information but we have had no updates," said one woman, who declined to give her name. "I hope someone can stand up and say something, but I know there is not much information yet," she added.
If confirmed, the crash would be the worst disaster in the history of Malaysia Airlines, which has had no major accidents for almost two decades. The Boeing 777 is also one of the world's safest planes: the first fatal crash in its history only came last July when an Asiana Airlines jet landed short of the runway in San Francisco, killing three of the 307 people on board.