KUALA LUMPUR: Several nations searched waters off Southeast Asia on Saturday after a Malaysia Airlines jet carrying 239 passengers disappeared and was presumed crashed, leaving stunned relatives demanding answers.

Contact with Flight MH370 was lost somewhere between Malaysia's east coast and southern Vietnam, but its fate remained a mystery more than 16 hours after it slipped off radar screens.

Air search operations were halted at nightfall, though ships continued searching, the airline said, adding that no trace of the passenger plane had been found as of late Saturday.

The flight was en route from Kuala Lumpur to Beijing, with 227 passengers and 12 crew members from 14 nations, the airline said.

Frustrated officials and passengers' families 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.

“We are looking at all possibilities but it is too soon to speculate,” Malaysian Prime Minister Najib Razak said, when asked whether terrorism could have been a factor.

The plane's disappearance triggered a search effort involving vessels from several nations with rival maritime claims in the tense South China Sea.

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

Vietnam, Malaysia and the Philippines said they threw aircraft and vessels into the effort, and Singapore dispatched an air force C130 transport plane to the region.

Najib said the US navy also had agreed to send planes to help.

Authorities will search “for as long as it takes,” Najib told reporters.

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

Contact lost after two hours

Contact with the aircraft was lost at 2:40 am Malaysian time, about two hours after take-off from Kuala Lumpur International Airport, the airlines' 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 ever for the widely used Boeing 777.

A 777-200 operated by South Korea's Asiana Airlines skidded off the runway in San Francisco in 2013, killing three.

Malaysia Airlines also 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.

Indonesia-based aviation analyst Gerry Soejatman said a “24-hour golden window” for search and rescue efforts was closing fast.

“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 153 Chinese passengers aboard the plane included an infant, while 38 Malaysians and seven Indonesians were aboard.

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

The pilot 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.”

Distraught relatives

Vietnamese state media quoted a naval official in the country as saying the plane had crashed into the sea off Vietnam's southern coast.

But hours later, Hanoi had offered no further information and the claim could not be confirmed.

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's 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.”

A deadly 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.