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.

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. Investigators in Malaysia are voicing scepticism that the airliner that disappeared early Saturday with 239 people on board was the target of an attack, US and European government sources close to the probe said.

Neither Malaysia's Special Branch, the agency leading the investigation locally, nor spy agencies in the United States and Europe have ruled out the possibility that militants may have been involved in downing Malaysia Airlines Flight MH370. But Malaysian authorities have indicated that the evidence so far does not strongly back an attack as a cause for the aircraft's disappearance, and that mechanical or pilot problems could have led to the apparent crash, the US sources said.

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 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.

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.

Meanwhile, the director of the Central Intelligence Agency said on Tuesday intelligence officials could not rule out terrorism as a factor in the disappearance of a Malaysian Airlines plane. "You cannot discount any theory, " CIA Director John Brennan said during rare public comments in Washington.

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 since it was reported missing on Saturday.

The head of international police agency Interpol said on Tuesday he did not believe the disappearance of a Malaysia Airlines plane at the weekend was a terrorist incident. Brennan, asked about the plane at a Council on Foreign Relations event, said terrorism could not yet be ruled out.