The missing Malaysia Airlines jet had its communication devices switched off and was deliberately flown back on its original course, the Malaysian Prime Minister, Najib Razak, said on Saturday.

He said the plane’s last known communication was at 8.11am on the day it disappeared - more than six hours after its last reported position in the South China Sea.

Mr Sri Najib named two possible "corridors’’ thousands of kilometres from the original search locations, in official statements that serve to further deepen the mystery surrounding the disappearance of MH370.

And flagging the possibility of some form of terrorist act, Mr Najib said: "In view of this latest development, the Malaysian authorities have refocused their investigation into the crew and passengers on board."

Mr Najib described how ‘‘primary radar data’’ showed how the plane turned from its north-easterly course, flew west back over Malaysia, then turned north west after reaching the Straits of Malacca on the country’s west coast.

‘‘These movements are consistent with deliberate actions by someone on the plane,’’ Mr Najib said.

He added: ‘‘The aircraft could now be in one of two possible corridors. A northern corridor stretching approximately from the border of Kazakhstan and Turkmenistan to northern Thailand.

‘‘Or, a southern corridor stretching approximately from Indonesia to southern Indian ocean.’’

Given the new locations, Mr Najib said Malaysia was ending its search operations in the South China sea, and reassessing the search operation, which has involved 14 countries, 43 ships and 58 aircraft.

Despite the deliberate nature of the flight’s movements, Mr Najib stopped short of confirming an AP media report citing Malaysian officials who said a hijacking was ‘‘conclusive’’.

"Despite media reports that the plane was hijacked I wish to be very clear that we are still investigating all possibilities," Mr Najib said. 

Mr Razak defended the relative silence from his government since the aircraft disappeared, saying that he had a responsibility to the families and the investigation to ‘‘only release information that has been corroborated’’.

He said ‘‘no words can describe the pain’’ the family and friends of those missing must be going through. He offered his thoughts and prayers.

Earlier in the day the News Corp chairman, Rupert Murdoch, tweeted one of the wilder theories about the plane when he wrote: ‘‘World seems transfixed by 777 disappearance.  Maybe no crash but stolen, effectively hidden, perhaps in Northern Pakistan, like Bin Laden.’’

One of the new corridors puts the plane on a passage towards Afghanistan.

A Malaysian government official involved in the investigation said on Saturday investigators have concluded that one or more people with significant flying experience hijacked and steered it off-course.

No motive has been established and no demands have been made known, and it is not yet clear where the plane was taken, said the official, who spoke on condition of anonymity because he was not authorised to brief the media. The official said that hijacking was no longer a theory.

"It is conclusive," he said.

His views were made public only minutes before the address by the Malaysian Prime Minister, Najib Razak.

The officials said evidence that led to the conclusion were signs that the plane's communications were switched off deliberately, data about the flight path and indications the plane was steered in a way to avoid detection by radar.

The Boeing 777's communication with the ground was severed a little less than one hour into the flight on March 8 from Kuala Lumpur to Beijing. Malaysian officials previously have said radar data suggest it may have turned back toward and crossed over the Malaysian peninsula after setting out on a north-eastern path toward the Chinese capital.

Earlier, an American official told the Associated Press investigators are examining the possibility of "human intervention" in the plane's disappearance, adding it may have been "an act of piracy".

While other theories are still being examined, the US official said key evidence suggested human intervention is that contact with the Boeing 777's transponder stopped about a dozen minutes before a messaging system on the jet quit. Such a gap would be unlikely in the case of an in-flight catastrophe.

The Malaysian official said only a skilled aviator could navigate the plane the way it was flown after its last confirmed location over the South China Sea. The official said it had been established with a "more than 50 percent" degree of certainty that military radar had picked up the missing plane after it dropped off civilian radar.

Mr Najib’s statements follow a series of US media reports relying on military radar data suggesting the plane had turned in mid flight.

Reuters reported military radar data suggested the jet was deliberately flown hundreds of kilometres off course, heightening suspicions of foul play among investigators, sources told Reuters on Friday.

Analysis of the Malaysia data suggests the plane, with 239 people on board, diverted from its intended northeast route from Kuala Lumpur to Beijing and flew west instead, using airline flight corridors normally employed for routes to the Middle East and Europe, sources familiar with investigations into the Boeing 777's disappearance said.

Two sources said an unidentified aircraft that investigators believe was Flight MH370 was following a route between navigational waypoints when it was last plotted on military radar off the country's northwest coast.

This indicates that it was either being flown by the pilots or someone with knowledge of those waypoints, the sources said.

The last plot on the military radar's tracking suggested the plane was flying toward India's Andaman Islands, a chain of isles between the Andaman Sea and the Bay of Bengal, they said.

The New York Times reported the flight experienced significant changes in altitude after it lost contact with ground control, climbing to 45,000 feet - above the approved limit for a Boeing 777-200 - and then descending unevenly to 23,000 feet.

AP and Alana Schetzer