THE co-pilot of a missing ­Malaysian jet spoke the last words heard from the cockpit, it has been revealed, as investigators consider suicide by the captain or first officer as one possible explanation for the disappearance.

No trace of Malaysia Airlines Flight MH370 has been found since it vanished on March 8 with 239 people aboard.

Investigators are increasingly convinced it was diverted perhaps thousands of miles off course by someone with deep knowledge of the Boeing 777-200ER and commercial navigation.

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A search unprecedented in its scale is now under way for the plane, covering a area stretching from the shores of the Caspian Sea in the north to deep in the southern Indian Ocean.

Airline chief executive Ahmad Jauhari Yahya told a news ­conference that the possibility of suicide was being investigated.

He also said it was unclear exactly when one of the plane's automatic tracking systems had been disabled, appearing to contradict the weekend comments of government ministers.

Suspicions of hijacking or ­sabotage had hardened further when officials said on Sunday the last radio message from the plane - an informal "all right, good night" - was spoken after the system, known as Acars, was shut down.

When asked who was believed to have spoken those words, Mr Ahmad Jauhari said: "Initial investigations indicate it was the co-pilot who basically spoke the last time it was recorded on tape."

That was a sign-off to air traffic controllers at 1.19am, as the Beijing-bound plane left Malaysian airspace.

The last transmission from the Acars system, a maintenance computer that relays data on the plane's status, had been received at 1.07am, as the plane crossed Malaysia's north-east coast and headed out over the Gulf of Thailand.

Mr Ahmad Jauhari said: "We don't know when the Acars was switched off after that. It was supposed to transmit 30 minutes from there, but that transmission did not come through."

Police and a multi-national ­investigation team may never know for sure what happened in the cockpit unless they find the plane, and that is a daunting challenge.

Satellite data suggests it could be anywhere in either of two vast corridors that arc through much of Asia: one stretching north from Laos to the Caspian, the other south from west of the Indonesian island of Sumatra into the southern Indian Ocean west of Australia.

Kazakhstan, at the end of the northern arc, said it had not detected any "unsanctioned use" of its air space on March 8.

China, which has been vocal in its impatience with Malaysian efforts, has called on its smaller neighbour to immediately expand and clarify the scope of the search. About two-thirds of the passengers aboard MH370 were Chinese.

Australian Prime Minister Tony Abbott said he had spoken to Malaysian counterpart Najib Razak by telephone and had offered more surveillance resources in addition to the two aircraft his country has already committed.

Mr Abbott told parliament: "He asked that Australia take responsibility for the search in the southern vector, which the Malaysian authorities now think was one possible flight path for this ill-fated aircraft. I agreed that we would do so."

Malaysian Acting Transport Minister Hishammuddin Hussein said diplomatic notes had been sent to all countries along the northern and southern search corridors, requesting radar and satellite information as well as land, sea and air search operations.

Malaysian authorities believe that, as the plane crossed the ­north-east coast and flew across the Gulf of Thailand, someone on board shut off its communications systems and turned west.

Police are checking the ­backgrounds of the pilots, flight and ground staff for any clues to a possible motive in what they say is now being treated as a criminal investigation.

Asked if pilot or co-pilot suicide was a line of inquiry, Mr Hishammuddin said: "We are looking at it." But he added it was only one of the possibilities under investigation.

Police special branch officers have searched the homes of the captain, 53-year-old Zaharie Ahmad Shah, and first officer Fariq Abdul Hamid, 27.