* Malaysia Airlines says "fearing the worst"
* Flight lost contact hour after takeoff, no distress signal or sign of bad weather
* KL-Beijing flight carried 227 passengers and 12 crew
* Two passengers, possibly more, were travelling under false identities
Malaysia Airlines said it was "fearing the worst" on Sunday for a plane carrying 239 people that went missing more than 24 hours ago, as the government said it was investigating four passengers who may have held false identity documents.
There were no reports of bad weather and no sign of why Flight MH370 would have vanished from radar screens off the coast of Vietnam about an hour after it took off from Kuala Lumpur bound for Beijing early on Saturday morning.
European officials said it appeared two people on board were using stolen passports and Malaysian Transport Minister Hishamuddin Hussein said authorities were also checking the identities of two other passengers.
"All the four names are with me," said Hishamuddin, who is also defence minister. "I have indicated to our intelligence agencies and I have also spoken to international intelligence agencies for assistance."
He said help was also being sought from the U.S. Federal Bureau of Investigation (FBI). However, an attack was only one of the possibilities being investigated.
"We are looking at all possibilities," he said. "We cannot jump the gun. Our focus now is to find the plane."
The Chinese official Xiamen Daily reported that one of the passengers who was supposed to be on the flight, according to the manifest, was at home in China. The name on the passport and the passport number did not match, according to the newspaper.
It was not immediately clear if the Chinese person's identity was among those being investigated.
There were no confirmed signs of the plane or any wreckage on Sunday, well over 24 hours after it went missing. Search operations continued through the night, officials said.
"In fearing for the worst, a disaster recovery management specialist from Atlanta, USA, will be assisting Malaysia Airlines in this crucial time," the airline said in a statement.
Vietnamese naval boats sent from the holiday island of Phu Quoc patrolled stretches of the Gulf of Thailand, searching for any wreckage, scouring the area where an oil slick was spotted by patrol jets just before nightfall on Saturday.
"Our two rescue boats have approached the two oil spills since 3 a.m. today but we haven't found any sign of the Malaysian plane yet. Other boats are ready to go to support if needed," Admiral Ngo Van Phat told Reuters.
There were no indications of sabotage nor claims of an attack. But the passenger manifest issued by the airline included the names of two Europeans - Austrian Christian Kozel and Italian Luigi Maraldi - who, according to their foreign ministries, were not on the plane.
A foreign ministry spokesman in Vienna said: "Our embassy got the information that there was an Austrian on board. That was the passenger list from Malaysia Airlines. Our system came back with a note that this is a stolen passport."
Austrian police had found the man safe at home. The passport was stolen two years ago while he was travelling in Thailand, the spokesman said.
The foreign ministry in Rome said no Italian was on the plane either, despite the inclusion of Maraldi's name on the list. His mother, Renata Lucchi, told Reuters his passport was lost, presumed stolen, in Thailand in 2013.
U.S. and European security officials said that there was no proof of foul play and there could be other explanations for the use of stolen passports.
A Malaysian official with knowledge of the investigation said the passengers being checked had all bought their tickets through China Southern Airlines, which was code-sharing the flight with Malaysia Airlines, the official said.
Passengers on board the flight included 20 employees of Austin, Texas-based chip maker Freescale Semiconductor Ltd . Twelve of the employees were from Malaysia and eight from China, the company said in a statement.
The 11-year-old Boeing 777-200ER, powered by Rolls-Royce Trent engines, took off at 12:40 a.m. (1640 GMT Friday) from Kuala Lumpur International Airport when it went missing without a distress call. Aboard were 227 passengers and 12 crew.
Flight MH370 last had contact with air traffic controllers 120 nautical miles off the east coast of the Malaysian town of Kota Bharu. Flight tracking website flightaware.com showed it flew northeast after takeoff, climbed to 35,000 ft (10,670 metres) and was still climbing when it vanished from tracking records.
A crash would likely mark the 777's second fatal incident in less than a year, and its deadliest since entering service 19 years ago. An Asiana Airlines Boeing 777-200ER crash-landed in San Francisco in July 2013, killing three passengers and injuring more than 180.
Boeing said it was monitoring the situation but had no further comment.
Paul Hayes, director of safety at Flightglobal Ascend aviation consultancy, said the flight would normally have been at a routine stage, having reached initial cruise altitude.
"Such a sudden disappearance would suggest either that something is happening so quickly that there is no opportunity to put out a mayday, in which case a deliberate act is one possibility to consider, or that the crew is busy coping with what whatever has taken place," he told Reuters.
A large number of planes and ships from several countries were scouring the area where the plane last made contact, about halfway between Malaysia and the southern tip of Vietnam.
"The search and rescue operations will continue as long as necessary," Malaysian Prime Minister Najib Razak told reporters. He said his country had deployed 15 air force aircraft, six navy ships and three coastguard vessels.
Besides the Vietnamese vessels, China and the Philippines also sent ships to the region to help, while the United States, the Philippines and Singapore dispatched military planes. China also put other ships and aircraft on standby.
"BIG RED FLAG"
The disappearance of the plane is a chilling echo of an Air France flight that crashed into the South Atlantic on June 1, 2009, killing all 228 people on board. It vanished for hours and wreckage was found only two days later.
John Goglia, a former board member of the National Transportation Safety Board, the U.S. agency that investigates plane crashes, said the lack of a distress call suggested that the plane either experienced an explosive decompression or was destroyed by an explosive device.
"It had to be quick because there was no communication," Goglia said. He said the false identities of the two passengers was "a big red flag".
If there were passengers on board with stolen passports, it was not clear how they passed through security checks.
International police body Interpol maintains a database of more than 39 million travel documents reported lost or stolen by 166 countries, and says on its website that this enables police, immigration or border control officers to check the validity of a suspect document within seconds. No comment was immediately available from the organisation.