AN international land and sea search for a missing Malaysian jet is covering an area the size of Australia but police and intelligence agencies have yet to establish a clear motive to explain its disappearance.

Investigators are convinced someone with deep knowledge of the Boeing 777-200ER and commercial navigation diverted Malaysia Airlines Flight MH370, carrying 12 crew and 227 mainly Chinese passengers, perhaps thousands of miles off its scheduled course from Kuala Lumpur to Beijing.

However, intensive background checks of everyone aboard have so far failed to find anyone with a known political or criminal motive to hijack or deliberately crash the plane.

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Malaysian Acting Transport Minister Hishammuddin Hussein said the "unique, unprecedented" search covered a total area of 2.24 million nautical miles from central Asia to the southern Indian Ocean.

Flight MH370 vanished from civilian air traffic control screens off Malaysia's east coast less than an hour after take-off early on March 8.

Investigators piecing together patchy data from military radar and satellites believe someone turned off the aircraft's identifying ­transponder and Acars system, which transmits maintenance data, and turned west, following a ­commercial aviation route towards India.

Malaysian officials have ­backtracked on the exact sequence of events. They are now unsure whether the Acars system was shut down before or after the last radio message from the cockpit, but said that did not make a material difference.

Mr Hishammuddin said: "This does not change our belief that up until the point at which it left military primary radar coverage, the aircraft's movements were consistent with deliberate action by someone on the plane."

China's ambassador to Malaysia said his country had carried out a detailed probe into its nationals aboard the flight and could rule out their involvement.

US and European security sources said efforts by various governments to investigate the backgrounds of everyone on the flight had not turned up links to militant groups or anything else that could explain the jet's disappearance.

A European diplomat in Kuala Lumpur also said trawls through the passenger manifest had come up blank.

One US report cited senior US officials as saying the first turn back to the west was likely programmed into the aircraft's flight computer, rather than being executed manually, by someone knowledgeable about aircraft systems.

However, Malaysia Airlines Chief Executive Ahmad Jauhari Yahya said that was speculation, adding: "Once you are in the aircraft, anything is possible."

A senior police officer said programs from a flight simulator in the home of pilot Zaharie Ahmad Shah, 53, included runways in the Maldives, Sri Lanka, Diego Garcia and southern India.

China, which is leading the search in the northern search corridor, said it had deployed 21 satellites to scour its territory.

John Young, of the Australian Maritime Safety Authority (AMSA) which is leading the southern search, said: "A needle in a haystack remains a good analogy."