AN investigation into the disappearance of a Malaysia Airlines jetliner is now focusing on a suspicion of foul play, as evidence suggests it was diverted hundreds of miles off course.

Military radar plotting suggests that an unidentified aircraft, which investigators suspect was missing Flight MH370, appeared to be following a commonly used route when it was last spotted early on Saturday, northwest of Malaysia.

The course - headed into the Andaman Sea - could only have been set deliberately, either by flying the Boeing 777-200ER manually or by auto-pilot.

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The last sighting of the aircraft on civilian radar came shortly before 1.30am, less than one hour after take-off from Kuala Lumpar, flying, as scheduled across the mouth of the Gulf of Thailand on the eastern side of peninsular Malaysia, heading towards Beijing.

However, Malaysia's air force chief has said an aircraft that could have been the missing plane was plotted on military radar at 2.15am, 200 miles off Malaysia's west coast.

The fact that the plane - which analysts are trying to identify as the MH370 or not - had lost contact with air traffic control and was invisible to civilian radar suggests someone on board had turned off its communication systems.

A source close to the investiation said inquiries were now focusing on the theory that someone who knew how to fly a plane diverted the flight hundreds of miles off course.

"What we can say is, we are looking at sabotage, with hijack still on the cards," said the source, a senior Malaysian police official.

Malaysian Transport Minister Hishammuddin Hussein said he could not confirm the last heading of the plane or if investigators were focusing on sabotage.

"A normal investigation becomes narrower with time as new information focuses the search, but in this case, the information has forced us to look further afield."

The latest radar evidence is consistent with the expansion of the search to the west of Malaysia.

However, if the jetliner did enter the Andaman Sea heading toward the Indian Ocean, a vast expanse with depths of more than 23,000 feet, the task faced by searchers becomes daunting. Winds and currents could shift surface debris tens of nautical miles within hours, dramatically widening the search area with each passing day.

There has been no trace of the plane nor any sign of wreckage as military aircraft and vessels of more than a dozen countries scour vast areas of south-east Asian seas.

"Ships alone are not going to get you that coverage, helicopters are barely going to make a dent in it and only a few countries fly [long-range search aircraft] P-3s," said William Marks, spokesman for the US Seventh Fleet.

"So this massive expanse of water space will be the biggest challenge," he added.

China, which had more than 150 citizens on board the missing plane, has deployed four warships, four coastguard vessels, eight aircraft and trained 10 satellites on a wide search area.

Chinese media have described the deployment as the largest Chinese rescue fleet ever assembled.

An already difficult search has been complicated in some areas by dense smoke caused by burning forest and farmland, which has enveloped much of Malaysia and spilled into the Strait of Malacca.