HANOI (AFP, REUTERS) - Rescuers from Vietnam, Malaysia, Singapore and China mounted an air and sea search on Saturday for a Malaysia Airlines jet that has gone missing over Southeast Asia, with grave fears for the 239 people on board.

Vietnam authorities said contact with Flight MH370 was lost near its airspace, but its exact location and fate remained a mystery more than 14 hours after it slipped off air-traffic control screens.

Frustrated officials and passenger's relatives struggled to make sense of the disappearance of the Boeing 777-200 which - like the Malaysian national carrier - has a solid safety record.

The airline 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.

Asked whether Malaysia was looking at possible terrorism, Transport Minister Hishammuddin Hussein said the government was "looking at all possibilities." "The plane lost contact near Ca Mau province airspace as it was preparing to transfer to Ho Chi Minh City air traffic control," the Vietnamese government said in a statement.

Ca Mau province is in southernmost Vietnam, next to the Cambodian border.

The airline said the plane's last known position was a point about 150 kilometres north of Malaysia's eastern coast.

The disappearance triggered a South China Sea search effort involving vessels from several nations with rival maritime claims to the region.

China, which has 153 of its nationals on the missing plane, said it had ordered maritime patrol vessels to begin scouring the area.

Vietnam's defence ministry launched a rescue mission, the government said, and a Malaysian maritime official said the country had sent several planes and vessels.

The Philippines said it was sending three navy patrol boats and a surveillance plane.

Singapore dispatched an air force C130 transporter on a "search and locate mission".

It was not clear if the nations were cooperating.

Overlapping claims to the South China Sea, a resource-rich region and vital shipping lane, have been a growing source of tensions between China and its neighbours.

Contact with the aircraft was lost at 2:40 am Malaysian time (1840 GMT Friday), about two hours after take-off from Kuala Lumpur International Airport, the carrier's CEO Ahmad Jauhari Yahya said.

"Our focus now is to work with emergency responders and authorities, and mobilise full support," he told a press conference, adding the airline's "thoughts and prayers" were with those affected.

If a crash is confirmed, it would be only the second fatal crash in the history of the Boeing 777, a popular wide-body model. A Boeing 777-200 operated by South Korea's Asiana Airlines skidded off the runway in San Francisco in 2013, killing three.

Indonesia-based aviation analyst Gerry Soejatman said the clock was ticking on a "24-hour golden window" for search and rescue efforts.

"You can't assume that there are no survivors, and if there are any, it is absolutely crucial that they are picked up within a day, or the chances of survival drops significantly," he said.

The plane carried 227 passengers and 12 crew members from 14 nationalities.

The 153 Chinese passengers included an infant, while 38 Malaysians and 7 Indonesians were also aboard, the airline said.

Six Australians, four French nationals, and three Americans including an infant, were also among those listed.

Malaysia Airlines has an admirable safety record. Its worst-ever crash occurred in 1977, when 93 passengers and seven crew perished in a hijacking and subsequent crash in southern Malaysia.

The pilot of MH370 had flown for the carrier since 1981, it said. The plane is more than 11 years old.

"This news has made us all very worried," Chinese Foreign Minister Wang Yi said in Beijing.

"We hope every one of the passengers is safe. We are doing all we can to get more details."

Hishammuddin, the Malaysian transport minister, said he had no confirmation on the plane's fate.

He said Malaysia was pressing Hanoi for details after state media quoted a Vietnamese naval official as saying it had crashed into the sea.

The lack of information sparked fury among anguished relatives in Beijing.

"They should have told us something before now," a visibly distressed man in his 30s said at a hotel where passengers' families were asked to gather.

"They are useless," another young man said of the airline. "I don't know why they haven't released any information."

At Kuala Lumpur International Airport, distraught family members trickled in to a designated waiting area for loved ones, escorted by authorities.

Hamid Ramlan, a 56-year-old Kuala Lumpur police officer, said his daughter and son-in-law were on the flight for an intended holiday in Beijing.

"My wife is crying. Everyone is sad. My house has become a place of mourning," he said. "This is Allah's will. We have to accept it." "Being a policeman over 33 years, this is my worst day."

An accident would be a huge blow for Malaysian Airlines, which has bled money for years as its struggles to fend off competition from rivals such as fast-growing Malaysia-based AirAsia.

Analysts have blamed poor management, government interference, and union resistance to reform of the airline.