KUALA LUMPUR (REUTERS) - 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 said.

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.

Malaysian authorities have said previously that Flight MH370 disappeared about an hour after it took off early Saturday from Kuala Lumpur bound for the Chinese capital, Beijing.

But a senior military officer who has been briefed on investigations said on Tuesday that the aircraft had made a detour to the west after communications with civilian authorities ended.

"It changed course after Kota Baru and took a lower altitude. It made it into the Malacca Strait," the officer said.

The Strait of Malacca, one of the world's busiest shipping channels, runs along Malaysia's west coast, while Kota Baru is on the northeast coast.

Such a detour would appear to undermine the theory that the aircraft suffered a sudden, catastrophic mechanical failure, as it would mean the plane flew at least 500 km after its last contact with air traffic control.

The plane's transponder and other tracking systems were either shut off or not functioning around the time that communications with air traffic control ended.

That would have prevented so-called secondary radar used by civilian authorities from identifying it, but not primary radar used by the military.

After the comments from the officer, a non-military source familiar with the investigations said the reported detour was one of several theories and was being checked.

A spokesman for the Malaysian prime minister's office said on Wednesday he had not been informed by the military of evidence showing the plane had recrossed the Malay Peninsula to reach the Malacca Strait.

"The people I checked with were not aware of that," the spokesman, Tengku Sariffuddin Tengku Ahmad, said.

He said all that had been confirmed to him was that military radar suggested it could have turned back from its scheduled flight path and the search of the Malacca Strait was in order to cover all possibilites.

At the time it lost contact with civilian air traffic control, the plane was roughly midway between Kota Bharu, to the northeast of Kuala Lumpur, and the southern tip of Vietnam, flying at 35,000 feet (10,670 metres).

Malaysia's Berita Harian newspaper quoted Air Force chief Rodzali Daud as saying the plane was last detected at 2:40 a.m. 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.

A huge international search operation 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, coast guard and civilian vessels from 10 nations have criss-crossed the seas off both coasts of Malaysia without success.

In the absence of any concrete evidence to explain the plane's disappearance, authorities have not ruled out anything.

Police have said they were investigating whether any passengers or crew on the plane had personal or psychological problems that might shed light on the mystery, along with the possibility of a hijacking, sabotage or mechanical failure.

"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 said at 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 in a statement.

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 apparently in a plane's cockpit.

The fact that at least two passengers on board the missing flight had used stolen passports has raised suspicions of foul play. But Southeast Asia is known as a hub for false documents that are also used by smugglers, illegal migrants and asylum seekers.

Interpol Secretary General Ronald Noble named the two men as Iranians, aged 18 and 29, who had entered Malaysia using their real passports before using the stolen European documents to board the Beijing-bound flight.

"The more information we get, the more we are inclined to conclude it is not a terrorist incident," Noble said.

The Malaysian police chief, Khalid, said the younger man, who he said was 19, appeared to be an illegal immigrant. His mother was waiting for him in Frankfurt and had been in contact with authorities, he said.

"We believe he is not likely to be a member of any terrorist group, and we believe he was trying to migrate to Germany," Khalid said, though he said he could not eliminate the possibility of a hijacking until the investigations were completed.

In Washington, the director of the CIA in Washington said intelligence officials could not rule out terrorism as a factor."You cannot discount any theory," CIA Director John Brennan said.

About two-thirds of the 227 passengers and 12 crew now presumed to have died aboard the plane were Chinese. Other nationalities included 38 Malaysians, seven Indonesians, six Australians, five Indians, four French and three Americans.

China has deployed 10 satellites using high-resolution earth imaging capabilities, visible light imaging and other technologies to "support and assist in the search and rescue operations", the People's Liberation Army Daily said.

US government officials from the National Transportation Safety Board and the Federal Aviation Administration have arrived in the region to provide "any necessary assistance" with the investigation, White House spokesman Jay Carney said in Washington.

Underlining the breadth of the search effort, an international body set up to detect possible breaches of a ban on nuclear tests said it was analysing infrasound data "for possible clues" on the missing flight.

The Vienna-based Preparatory Commission of the Comprehensive Nuclear Test Ban Treaty Organisation, which has a global network of monitoring stations, said standard reports from its International Data Centre "did not reveal anything that could aid in the search for the missing MH370 plane".

The Boeing 777 has one of the best safety records of any commercial aircraft in service. Its only previous fatal crash came on July 6 last year when Asiana Airlines Flight 214 struck a seawall on landing in San Francisco, killing three people.

US planemaker Boeing has declined to comment beyond a brief statement saying it was monitoring the situation.