Confusion surrounds the fate of the missing Malaysian airliner as aviation chiefs refused to confirm military claims that the Boeing 777 changed course over the sea.

Military officials said radar showed the plane crossed Malaysia and reached the Strait of Malacca - hundreds of miles from its last position recorded by civilian authorities.

The country's civilian aviation chief Azharuddin Abdul Rahman said he could neither confirm nor deny the military's remarks. That suggests disagreement or confusion at the highest level over where the plane is most likely to have ended up.

"There is a possibility of an air turn-back. We are still investigating and looking at the radar readings," Mr Rahman said. It is possible that the radar readings are not definitive, especially if the plane was malfunctioning.

The developments added mystery into one of most puzzling aviation incidents of recent time, and it has raised questions about why the Malaysia Airlines flight was not transmitting signals detectable by civilian radar, why its crew was silent about the course change and why no distress calls were sent after it turned back.

Many experts have been working on the assumption there was a catastrophic event on the flight - such as an explosion, engine failure, terrorist attack, turbulence, pilot error or even suicide. The director of the CIA said in Washington that he still would not rule out terrorism.

Flight MH370, carrying 239 people, took off from Kuala Lumpur at 12.41am on Saturday, bound for Beijing. Authorities initially said its last contact with ground controllers was less than an hour into the flight at a height of 35,000ft, when the plane was somewhere between the east coast of Malaysia and Vietnam.

But local newspaper Berita Harian quoted Malaysia's air force chief, General Rodzali Daud, as saying that radar at a military base had tracked the jet as it changed its course, with the final signal at 2.40am showing the plane to be near Pulau Perak at the northern approach to the Strait of Malacca, a busy waterway that separates the western coast of Malaysia and Indonesia's Sumatra island. It was flying slightly lower, at around 29,528ft, he said.

"After that, the signal from the plane was lost," he was quoted as saying.

Authorities had said earlier the plane might have tried to turn back to Kuala Lumpur, but they expressed surprise it would do so without informing ground control.

The search was initially focused hundreds of miles to the east, in waters off Vietnam, with more than 40 planes and ships from at least 10 nations searching the area without finding a trace of the missing aircraft.

Malaysia Airlines said in a statement yesterday that search-and-rescue teams had expanded their scope to the Strait of Malacca. An earlier statement said the western coast of Malaysia was "now the focus", but the airline subsequently said that phrase was an oversight.

Mr Rahman said the search remained "on both sides" of Malaysia.

Attention is likely to focus on the condition of the Boeing's electronic systems as it charted its new course back towards and then across Malaysia.

A radar antenna on the ground sends electromagnetic waves that reflect from the surface of an aircraft and almost instantly return, allowing controllers to calculate how far away a plane is. The antenna is mounted on a rotating platform, sending and receiving signals 360 degrees across the sky, enabling the plane's direction to be tracked by constant sweeps.

The system has limitations: military and civilian air traffic controllers know something is moving through the air but might not know what it is. So planes were outfitted with transponders that can send a unique signal back to the radar station, which can differentiate them from other aircraft. From this signal, controllers can tell the flight number, heading, speed and altitude.

Radar stations at airports are designed to track planes up to about 60 miles, to help sequence and space landing aircraft. Another series of stations called air route surveillance radar can track planes 200-250 miles away, depending on weather and the age of the technology. Station locations are selected to allow for a slight overlap so planes in heavy-traffic areas are never out of reach of radar.

While radar blackspots can exist, experts said the plane's transponders normally would have been emitting signals that would have been picked up by civilian radar. The fact that it apparently was not detected suggests they were either disabled or switched off. Planes with no transponders can still be tracked by radar.

Low-flying planes can sometimes avoid radar detection. There is no set height they must be under, but the farther away they are from a radar station, the higher they can be because of the angle of the radar antenna and the curvature of the Earth.

Malaysian police chief Khalid Abu Bakar, who has been ordered to look at possible criminal aspects in the disappearance of Flight MH370, said hijacking, sabotage and issues related to the pilots' psychological health were all being considered.

An Australian TV station reported that the first officer on the missing plane, Fariq Abdul Hamid, had invited two women into the cockpit during a flight two years ago. One of the women, Jonti Roos said she and a friend were allowed to stay in the cockpit during the entire one-hour flight on December 14, 2011, from Phuket, Thailand, to Kuala Lumpur. She said the arrangement did not seem unusual to the plane's crew.

"Throughout the entire flight, they were talking to us and they were actually smoking throughout the flight," said Ms Roos.

Malaysia Airlines said it took the allegations very seriously, which it said it was not able to confirm, adding: "We are in the midst of a crisis, and we do not want our attention to be diverted."

Malaysian and international police authorities said two people who boarded Flight MH370 with stolen passports were Iranians who had bought tickets to Europe, where they were planning to migrate. Their presence on the flight had raised speculation of a possible terrorist link.

Mr Bakar said investigators had determined one was a 19-year-old Iranian, Pouria Nourmohammadi Mehrdad. "We believe he is not likely to be a member of any terrorist group," Mr Bakar said.

Interpol identified the second man as Seyed Mohammed Reza Delavar, a 29-year-old Iranian, and released an image of the two boarding at the same time. Interpol secretary general Ronald K Noble said the two men travelled to Malaysia on their Iranian passports, then apparently switched to stolen Austrian and Italian documents.

CIA director John Brennan said in Washington that Malaysian authorities "are looking very carefully at what went wrong; you know, if these individuals got on to the plane with these stolen passports, why they were not aware of it".

He also said there has been "a lot of speculation right now - some claims of responsibility that have not been, you know, confirmed or corroborated at all. We are looking at it very carefully".

Asked if terrorism could be ruled out, Mr Brennan replied, "No, I wouldn't rule it out. Not at all."

Gen Daud later denied the remarks attributed to him that the plane was tracked by radar to the Malacca strait.

He said in a statement that the military believes the 777 might have turned back, and noted it first said that on Sunday.