Nearly 240 people are feared dead after air force jets spotted two large oil slicks where a passenger plane reportedly crashed into the sea off Vietnam.
The Malaysia Airlines Boeing 777-200 was flying from Kuala Lumpur to Beijing when it vanished around two hours into the flight.
The two slicks - both between six miles (10km) and nine miles (15km) long - were seen by two military jets off the south coast of Vietnam.
An information screen at Kuala Lumpur International Airport A Vietnamese government statement said they were consistent with the kind of spills caused by fuel from a crashed airliner.
Earlier, the country's state media quoted a navy official as saying Flight MH370 crashed near Tho Chu island at around 2.40am local time.
All ships in the local area have been sent to the scene to look for survivors.
Distraught relatives and friends wait at Beijing International Airport The US Navy is helping teams from Vietnam, Malaysia and Singapore search the vast waters in the Gulf of Thailand, between Malaysia and Vietnam.
Malaysia's Transport Minister Seri Hishammuddin said he was "looking at all possibilities" including a potential terror attack.
Malaysia Airlines said in a statement: "The families of all passengers on board MH370 are being informed.
The plane is said to have crashed near Tho Chu island, in southern Vietnam "At this stage, our search and rescue teams from Malaysia, Singapore and Vietnam have failed to find evidence of any wreckage."
The plane, carrying 239 people, disappeared from radar at 1.30am (5.30pm UK time) about 85 miles (135km) north of the Malaysian city of Kuala Terengganu.
Journalist Leo Lewis, at Beijing airport, told Sky News families were waiting anxiously for news of their relatives.
He said there were "scenes of considerable distress" and "increasing irritation" because of a lack of information about their fate.
Family members have complained of a lack of information Malcolm Moore, the Daily Telegraph's Beijing Correspondent, said the relatives have been taken by bus to a hotel in the centre of Beijing.
He told Sky News: "There has been no official confirmation (about what has happened), but it's looking increasingly grim."
The plane left Kuala Lumpur at 12.41am local time and was due to arrive in Beijing at 6.30am local time.
Flight MH370 marked in red on Beijing airport's information board Malaysia Airlines said the flight was carrying 227 passengers, including two infants, and 12 crew members.
They were from 14 countries - 152 plus one baby from China, 38 from Malaysia, 12 from Indonesia, seven from Australia, three plus one baby from the US, three from France, two from New Zealand, two from Ukraine, two from Canada, one from Russia, one from Italy, one from Taiwan, one from Netherlands, and one from Austria.
Chinese Prime Minister Li Keqiang has urged Malaysia to "quickly and vigorously push search and rescue work" for the missing plane, state news agency Xinhua said.
Beijing has sent two maritime rescue ships to the South China Sea to help in the search and rescue work.
Relatives of those on board await news at Beijing airport Chinese Foreign Minister Wang Yi said: "This news has made us all very worried. We hope every one of the passengers is safe."
The aircraft was piloted by Captain Zaharie Ahmad Shah, a 53-year-old Malaysian with a total of 18,365 flying hours. He joined the company in 1981.
"Our thoughts and prayers are with all affected passengers and crew and their family members," Malaysia Airlines CEO Ahmad Jauhari Yahya said in a statement.
A Boeing 777 crash landed in San Francisco The airline said it would provide "regular updates" on the situation and has set up the phone line +603 7884 1234 for concerned members of the public.
Aviation expert David Learmount told Sky News the Boeing 777-200's safety record is "absolutely superb".
He said the plane would have been travelling at cruising height (around 35,000ft) and even if both engines had failed, it would have given the pilots plenty of time to make a distress call.
The "simple solution" would be to presume there was a bomb on board, "but there's no suggestion of that," he said.
Malaysia Airlines has 15 Boeing 777-200 jets in its fleet of about 100 planes.
There has been one fatal accident involving a Boeing 777 since the jet entered service in 1995.
Last summer, an Asiana Airlines Boeing 777 crash landed in San Francisco, killing three passengers.
Boeing said it was aware of reports that the Malaysia Airlines plane has gone missing and was monitoring the situation, but had no further comment.
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