Nearly 240 people are feared dead after two large oil slicks were spotted 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 parallel slicks - both between 10 miles (16km) and 12 miles (19km) long and 500 metres apart - were seen by two Vietnamese air force jets off the south coast of Vietnam.
A Vietnamese government statement said they were consistent with the kind of spills caused by fuel from a crashed airliner.
Rescue boats are being sent to the area from the nearby island of Phu Quoc to look for survivors.
Earlier, the country's state media quoted a navy official as saying Flight MH370 crashed near Tho Chu Islands at around 2.40am local time.
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.
"At this stage, our search and rescue teams from Malaysia, Singapore and Vietnam have failed to find evidence of any wreckage."
US officials told NBC News they have not ruled out terrorism after it was discovered two passengers were travelling on stolen passports
The airline listed one of the passengers on the plane as a 37-year-old Italian called Luigi Maraldi.
However Mr Maraldi has contacted his parents to say he was not on the airliner.
He had his passport stolen in Thailand several months ago, leaving questions over who used his passport to board the plane and whether that has anything to do with the airliner's disappearance.
Mr Miraldi's father said his son's passport had been stolen a year and a half ago while he was travelling in Thailand.
"He deposited it with a car rental agency and when he returned the car it was gone," Walter Maraldi said.
Another passenger was using a passport belonging to Austrian citizen Christan Kozel. He is listed as one of the passengers although he has been confirmed as safe and well by authorities.
He said his passport was also stolen in Thailand when he visited two years ago.
The plane 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.
A woman whose mother was on the plane waited for hours without any news and expressed her anger at the way the airline is handling the incident.
Describing the scene inside the room where some 500 relatives are waiting for news, she said: "They used a bus to bring us here, in this hotel. They only give us bread, biscuits and water, that's all.
"There is nobody from the airline, not even one person. Every time I ask 'where is the airline staff; what's the latest?' They say, 'we are only volunteers, we're hotel staff, we're not from the airline'."
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.
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 from the US, three from France, two from New Zealand, two from Ukraine, two from Canada, one from Russia, one from Taiwan, and one from the Netherlands.
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.
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.
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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