Two passengers on the missing Malaysia Airlines Flight MH370 were travelling on stolen passports and bought one-way tickets, it has been confirmed, as authorities, including the FBI, investigate whether the disaster was the result of a terrorist act.

The aircraft, travelling from Kuala Lumpur to Beijing and carrying 239 people, went missing well into its flight Saturday morning. Nearly 48 hours after the last contact with Flight MH370, mystery still surrounds its fate.

Whatever happened to the Boeing 777 high above the South China Sea, it was quick and gave the pilots no time to issue a mayday, although there were reports that another Malaysia Airlines pilot flying ahead of the missing flight had managed to contact the plane at the request of air traffic control authorities.

There were also suggestions from the Malaysian Maritime Enforcement Agency that the plane may have tried to turn around, after the agency spotted oil slicks about 20 nautical miles south of the plane's last point of contact.

Neither of the two Europeans whose passports had been stolen was on the aircraft. The Italian, Luigi Maraldi, 37, was travelling in Thailand and the Austrian, Christian Kozel, 30, was located in his homeland.

Both men's passports were stolen in Phuket, a holiday island in Thailand.

Mr Maraldi was surprised to see his name on the passenger manifest because he is holidaying in Thailand.

He rang his father, Walter in Cesena, Italy, according to Italian press reports.

"Ciao, dad," he said. "Have you heard the news about the missing plane? Don’t worry, it wasn’t me who was listed as a passenger on board. I don’t know why my name is on the list, or what happened but I’m ok. I’m in Thailand."

On an earlier trip to Phuket in August last year Mr Maraldi had his passport stolen from a car rental agency.

According to media reports quoting Austrian authorities Mr Kozel had his passport stolen in the same part of Phuket 18 months earlier.

Both Mr Maraldi are Mr Kozel reportedly have average builds and wear glasses.

The New York Times reported on Sunday the two passengers who used the fraudulently obtained passports bought one-way tickets issued last week at the same travel agency in a shopping mall in the Thailand beach resort of Pattaya, according to electronic booking records.

A telephone operator on a China-based KLM hotline on Sunday confirmed that "Maraldi" and "Kozel" were both booked to leave Beijing on a KLM flight to Amsterdam on March 8. Maraldi was then to fly to Copenhagen, Denmark, on KLM on March 8, and Kozel to Frankfurt, Germany, on March 8.

She said since the pair booked the tickets through China Southern Airlines, she had no information on where they bought them.

A woman who answered the phone at the travel agency said she was "too busy to talk".

It is unclear how the men travelled south to Malaysia to board the flight on Saturday. In Beijing, each man was to continue on to separate European cities, according to the electronic records, the Times reported. As transit passengers, they would not have needed Chinese visas.

Security experts in Asia said the use of false travel documents was a persistent problem in the region, but differed on the significance of the two stolen passports to the investigation.

Xu Ke, a lecturer at the Zhejiang Police College in eastern China who studies aviation safety and hijackings, and has advised the Chinese authorities, said the two men might have been illegal migrants.

But Steve Vickers, the chief executive of a Hong Kong-based security consulting company that specialises in risk mitigation and corporate intelligence in Asia, said the presence of at least two travellers with stolen passports aboard a single jet was rare and a potential clue.

"It is fairly unusual to have more than one person flying on a flight with a stolen passport," said Mr Vickers, who warned publicly a month ago that stolen airport passes and other identity documents in Asia merited a crackdown.

"The future of this investigation lies in who really checked in and what they looked like," he added.

Malaysia's Department of Civil Aviation director general Azharuddin Abdul Rahman, said investigators were reviewing video footage of the passengers, including their check-in bags.

Malaysia's acting Transport Minister Hishammuddin Hussein said on Sunday investigators were checking the entire passenger list and that counter-terrorism units and the FBI had been informed.

At one stage on Sunday, Malaysian authorities said there were four passengers travelling on suspicious documents, but they later revised that back to two.

Asked whether this was a security lapse, Mr Hishammuddin said: "Let us not jump to conclusions and make wild speculation."

However, terrorism expert Greg Barton, of Monash University's School of Political and Social Inquiry, said modern aircraft did not just vanish from the sky and, when they did, a bomb was almost always the first suspect.

"Things will become clearer once wreckage or debris is found," he said.

"Beijing's reticence at linking the disappearance with terrorists is noticeable, but if debris or a black box indicates a mid-air explosion, expect the Uighurs [a Muslim people from the restive far western Chinese region of Xinjiang] to come into contention."

An Interpol spokeswoman said a check of all documents used to board the plane had revealed more "suspect passports" that were being further investigated. She was unable to say how many, or from which country or countries.

Malaysia's state news agency quoted Home Minister Ahmad Zahid Hamidi as saying the passengers using the stolen European passports were of "Asian" appearance, and criticising border officials who let them through.

"I am still perturbed. Can't these immigration officials think? Italian and Austrian [passport holders] but with Asian faces," he was quoted as saying late on Sunday.

Contact made briefly

According to Malaysia's New Straits Times newspaper, contact was briefly made with the aircraft before it vanished. It was being flown by Captain Zaharie Ahmad Shah, a highly experienced pilot, and first officer Fariq ab Hamid.

"We managed to establish contact with MH370 just after 1.30am and asked them if they have transferred into Vietnamese airspace," a pilot on another Malaysia Airlines flight told the New Straits Times.

"The voice on the other side could have been either Captain Zaharie or Fariq, but I was sure it was the co-pilot.

"There were a lot of interference ... static ... but I heard mumbling from the other end. That was the last time we heard from them, as we lost the connection," he said.

Vietnamese air force planes spotted two large oil slicks close to where the plane was presumed to have gone down in pitch darkness.

The slicks were the first possible indication that the aircraft had crashed.

Passengers and crew came from at least 14 countries and included six Australians - Robert and Catherine Lawton, Rodney and Mary Burrows and Gu Naijun and Li Yuan - and two New Zealanders. There were 154 Chinese nationals, 38 Malaysians, seven Indonesians, five Indians, four French citizens and three Americans.

Malaysia, China, the Philippines, US, Singapore and Australia have deployed ships and planes to the search area, which was widened after radar signals indicated the plane may have turned back.

Malaysian Air Force General Rodzali Daud said this theory was based on military radar.

"We looked back at the records – there was an indication the plane may have turned back," he said.

The Malaysian maritime agency's director-general Amdan Kurish, who joined in the search, said his team spotted "two or three" patches of yellowish oil slick about 16 kilometres long.

"A ship has been dispatched to the location of the slick to take samples so we could test whether the oil is from a plane," Mr Amdan said.

Even so, no trace of the aircraft has yet been found, and this failure has prompted speculation about what caused the disappearance. Apart from terrorism, other possibilities include airframe failure, bad weather, pilot disorientation, engine failure, hijacking, pilot suicide and being shot down.

Modern aircraft 'incredibly reliable'

David Learmount, operations and safety editor at Flightglobal media, said modern aircraft were "incredibly reliable and you do not get some sudden structural failure in flight – it just doesn't happen".

Malaysia Airlines has a good safety record, as does the Boeing 777. Boeing has manufactured 1030 of the aircraft and they had not had a fatal crash in their 19-year history until an Asiana Airlines plane crashed in San Francisco last year, killing three passengers.

However, the missing Malaysia Airlines 777 is reportedly the same aircraft that crashed into the tail of another plane while taxiing at Shanghai's Pudong International Airport in August 2012.

An independent accident-tracking website, Aviation Safety Network, listed the accident and claimed damaged suffered by the Boeing 777 was "substantial ... the tip of the wing of the Malaysia Airlines Boeing 777 was broken off and hung on the tail of the China Eastern Airbus 340-600, according to pictures posted by passengers on the internet".

At a news conference in Beijing on Sunday, Ignatius Ong, chief executive of Malaysia Airlines subsidiary Firefly airlines, said the plane was last inspected 10 days ago and was "in proper condition".

Australian Prime Minister Tony Abbott would not speculate on whether terrorism was involved in the crash.

He has offered Malaysia two Australian aircraft to assist with the search.

"This afternoon I spoke to Malaysian Prime Minister Najib Razak to convey Australia's condolences on the loss of Malaysia Airlines Flight 370 and offer our assistance with the search for the missing aircraft," he said in a statement on Sunday.

"On behalf of Australia, I offered two RAAF P-3C maritime surveillance aircraft to help with the search for the missing aircraft.

"The P-3C Orion is a long-range maritime surveillance aircraft ideally suited to this task."

Mr Najib accepted the offer and the first aircraft was due to depart from Darwin on Sunday night.

The disappearance of Flight MH370 paralleled the loss of Air France Flight 447, an Airbus 300, travelling from Rio de Janeiro to Paris in June 2009. It fell into the Atlantic, killing all 216 passengers and 12 crew.

A report the following year found a combination of technical faults and human error led to the crash.

Heavy turbulence caused air-speed sensors to malfunction while the captain was taking a rest break and the plane began to stall.

A spokeswoman for the Department of Foreign Affairs and Trade said the families of Australians listed on the flight had been contacted.

"Australian consular officials are in urgent and ongoing contact with local authorities and with Malaysia Airlines including on efforts to locate the missing flight," she said.