Planes and ships from south-east Asian states have joined forces to search the South China Sea for a Malaysia Airlines jet, missing with 239 people on board.
Flight MH370 vanished at 18:40 GMT Friday (02:40 local time Saturday) after leaving Kuala Lumpur for Beijing.
It had been expected to land in the Chinese capital at 22:30 GMT.
The aerial search has now been halted for the night but operations at sea continue. No wreckage has been reported by the airline.
Distraught relatives and loved ones of those aboard are being given assistance at the airports.
"We are doing everything in our power to locate the plane," Malaysian Transport Minister Hishammuddin Hussein told reporters in Kuala Lumpur.
Malaysia Airlines chief executive Ahmad Jauhari Yahya said the focus was on helping the families of those missing. He said that 80% of the families had been contacted.
The plane went off the radar south of Vietnam, according to a statement on the Vietnamese government website.
Its last known location was off the country's Ca Mau peninsula although the exact position was not clear, it said.
The Boeing B777-200 aircraft was carrying 227 passengers, including two children, and 12 crew members.
'Very worried'
Malaysia's military said a second wave of helicopters and ships had been despatched after an initial search revealed nothing. The US has agreed to help with its aircraft too, Malaysian Prime Minister Najb Razak said.
Territorial disputes over the South China Sea were set aside temporarily as China dispatched two maritime rescue ships and the Philippines deployed three air force planes and three navy patrol ships.
Singapore is also involved, while Vietnam sent aircraft and ships and asked fishermen in the area to report any suspected sign of the missing plane.
"In times of emergencies like this, we have to show unity of efforts that transcends boundaries and issues," said Lt Gen Roy Deveraturda, commander of the Philippine military's Western Command.
The passengers were of 14 different nationalities, Mr Jauhari said.
Among them were 152 Chinese nationals, 38 Malaysians, 12 people from Indonesia and six from Australia.
The pilot was Capt Zaharie Ahmad Shah, 53, who joined Malaysia Airlines in 1981, Mr Yahya said.
A Vietnamese navy official told the BBC the plane had gone missing within Malaysian maritime territory.
Friends and relatives expecting to meet passengers from the flight in Beijing were instructed to go to a nearby hotel where officials were meant to be on hand to provide support.
"They should have told us something before now," a visibly distressed man in his thirties told AFP news agency at the hotel.
"They are useless," another young man said of the airline. "I don't know why they haven't released any information."
In Kuala Lumpur, Hamid Ramlan, a 56-year-old police officer, said his daughter and son-in-law had been on the flight for an intended holiday in Beijing.
"My wife is crying," he said. "Everyone is sad. My house has become a place of mourning. This is Allah's will. We have to accept it."
The plane had been flying at an altitude of 35,000ft (10,700m) and the pilots had not reported any problems with the aircraft, Fuad Sharuji, Malaysia Airlines' vice-president of operations control, told CNN.
Malaysia's national carrier is one of Asia's largest, flying nearly 37,000 passengers daily to some 80 destinations worldwide.
The route between Kuala Lumpur and Beijing has become more and more popular as Malaysia and China increase trade, says the BBC's Jennifer Pak in Kuala Lumpur.
The Boeing 777 had not had a fatal crash in its 20-year history until an Asiana plane came down at San Francisco airport in July of last year.
Three teenage girls from China died in that incident.
