Confusion as search for jet expands

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.

If the plane had made such a detour it would undermine the theory that it suffered a sudden, catastrophic mechanical failure, as it would mean it flew at least 500 km (350 miles) after its last contact with air traffic control.

Special: Malaysia Airlines plane missing

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.

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 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 she and her friend were invited to fly in the cockpit by Fariq and the pilot between Phuket, Thailand, and Kuala Lumpur in December 2011.

"I thought that they were highly skilled and highly competent and since they were doing it that it was allowed," Roos said. "I want to make it clear, at no point did I feel we were in danger or that they were acting irresponsibly."

Hugh Dunleavy, the commercial director of Malaysia Airlines, said there was no reason to blame the crew.

"We have no reason to believe that there was anything, any actions, internally by the crew that caused the disappearance of this aircraft," he said.

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.