KUALA LUMPUR - Malaysia's military believes a jetliner missing for almost four days turned and flew hundreds of kilometres to the west after it last made contact with civilian air traffic control off the country's east coast, a senior officer told Reuters on Tuesday.

In one of the most baffling mysteries in recent aviation history, a massive search operation for the Malaysia Airlines Boeing 777-200ER has so far found no trace of the aircraft or the 239 passengers and crew.

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Malaysian authorities have previously said flight MH370 disappeared about an hour after it took off from Kuala Lumpur for the Chinese capital Beijing.

"It changed course after Kota Bharu and took a lower altitude. It made it into the Malacca Strait," the senior military officer, who has been briefed on investigations, told Reuters.

That would appear to rule out sudden catastrophic mechanical failure, as it would mean the plane flew around 500 km (350 miles) at least after its last contact with air traffic control, although its transponder and other tracking systems were off.

A non-military source familiar with the investigations said the report was one of several theories and was being checked.

Lost contact

At the time it lost contact with civilian air traffic control, the plane was roughly midway between Malaysia's east coast town of Kota Bharu and the southern tip of Vietnam, flying at 35,000 ft (10,670 metres).

The Strait of Malacca, one of the world's busiest shipping channels, runs along Malaysia's west coast.

Malaysia's Berita Harian newspaper quoted air force chief Rodzali Daud as saying the plane was last detected at 2.40 am by military radar near the island of Pulau Perak at the northern end of the Strait of Malacca. It was flying about 1,000 metres lower than its previous altitude, he was quoted as saying.

There was no word on what happened to the plane thereafter.

The effect of turning off the transponder is to make the aircraft inert to secondary radar, so civil controllers cannot identify it. Secondary radar interrogates the transponder and gets information about the plane's identity, speed and height.

It would however still be visible to primary radar, which is used by militaries.

Police had earlier said they were investigating whether any passengers or crew on the plane had personal or psychological problems that might explain its disappearance, along with the possibility of a hijack, sabotage or mechanical failure.

There was no distress signal or radio contact indicating a problem and, in the absence of any wreckage or flight data, police have been left trawling through passenger and crew lists for potential leads.

"Maybe somebody on the flight has bought a huge sum of insurance, who wants family to gain from it or somebody who has owed somebody so much money, you know, we are looking at all possibilities," Malaysian police chief Khalid Abu Bakar told a news conference.

"We are looking very closely at the video footage taken at the KLIA (Kuala Lumpur International Airport), we are studying the behavioural pattern of all the passengers."

The airline said it was taking seriously a report by a South African woman who said the co-pilot of the missing plane had invited her and a female travelling companion to sit in the cockpit during a flight two years ago, in an apparent breach of security.

"Malaysia Airlines has become aware of the allegations being made against First Officer Fariq Ab Hamid which we take very seriously. We are shocked by these allegations. We have not been able to confirm the validity of the pictures and videos of the alleged incident," the airline said.

The woman, Jonti Roos, said in an interview with Australia's Channel Nine TV that she and her friend were invited to fly in the cockpit by Hamid and the pilot between Phuket, Thailand and Kuala Lumpur in December 2011. The TV channel showed pictures of the four in the cockpit.

A huge search operation for the missing plane has been mostly focused on the shallow waters of the Gulf of Thailand off Malaysia's east coast, although the Strait of Malacca has been included since Sunday.

Navy ships, military aircraft, helicopters, coastguard and civilian vessels from 10 nations have criss-crossed the seas off both coasts of Malaysia without success.