Missing Boeing 777 mystery: Malaysia not sure which way jet was headed

That would place its last confirmed position closer to Malaysia than has previously been publicly disclosed.

Fathur said Malaysian authorities have determined four blocks to be searched in the strait, which Indonesia was assisting in.

Vietnam's Deputy Transport Minister Pham Quy Tieu was quoted by the Laborer Newspaper as telling reporters that operations had been scaled down following the air force chief's reported remarks, while Vietnam awaited confirmation.

But Lt. Gen. Vo Van Tuan, deputy chief of staff of Vietnamese People's Army, said this was not the case, and that efforts were being intensified. It wasn't immediately possible to clear up the conflicting accounts.

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

Aviation experts said they were becoming more uncertain about what most likely happened to the airliner.

Some said a major power outage was an unlikely explanation for why the aircraft's transponder and communications system were apparently not functioning at the time it was reportedly detected by Malaysian military radar flying back toward Malaysia.

With a power catastrophe so large that the various back-up systems, independent power supplies and built-in redundancies could not cope, the aircraft would be barely able to fly, said Jason Middleton, professor of aviation at the University of New South Wales.

Yet Middleton said if one or more passengers overpowered the pilots to take control of the plane, they would need training to switch off the transponder and other systems to ensure the jet was able to fly undetected.

“It's stretching belief a little bit that someone's going to be capable enough in the (777) to do all that,” Middleton said.

“It's a very curious outcome, you just can't rule out the possibility that the captain or the first officer have gone crazy,” he said.