KUALA LUMPUR, March 19 (Reuters) - Investigators probing the disappearance of a Malaysia Airlines jetliner with 239 people on board believe it most likely flew into the southern Indian Ocean, a source close to the investigation said on Wednesday.

No wreckage has been found from Flight MH370, which vanished from air traffic control screens off Malaysia's east coast at 1:21 a.m. local time on March 8 (1721 GMT March 7), less than an hour after taking off from Kuala Lumpur bound for Beijing.

An unprecedented search for the Boeing 777-200ER is under way involving 26 nations in two vast search "corridors": one arcing north overland from Laos towards the Caspian Sea, the other curving south across the Indian Ocean from west of Indonesia's Sumatra island to west of Australia.

"The working assumption is that it went south, and furthermore that it went to the southern end of that corridor," said the source, who spoke on condition of anonymity.

The view is based on the lack of any evidence from countries along the northern corridor that the plane entered their airspace, and the failure to find any trace of wreckage in searches in the upper part of the southern corridor.

Some sources involved in the investigation have voiced fears it could be drifting towards deadlock due to the reluctance of countries in the region to share militarily sensitive radar data or allow full access to their territory.

"These are basically spy planes, that's what they were designed for," said one source close to the investigation, explaining the hesitance of some nations to allow maritime surveillance aircraft into their waters.

Malaysia's Acting Transport Minister Hishammuddin Hussein, who is in charge of the operation, told a news conference that: "The search for MH370 involves diplomatic, technical and logistical challenges."

NEVER FOUND?

China, which is leading the northern corridor search with Kazakhstan, said it had not yet found any sign of the aircraft crossing into its territory.

Malaysian and U.S. officials believe the aircraft was deliberately diverted perhaps thousands of miles off course, but an exhaustive background search of the passengers and crew aboard has not yielded anything that might explain why.

Last week, a source familiar with official U.S. assessments said it was thought most likely the plane flew south, where it presumably would have run out of fuel and crashed into the sea.

If it did indeed end up in the southern Indian Ocean, one of the remotest places on Earth and also one of the deepest seas, it increases the chance it may never be found - and investigators may never know for sure what happened on board.

Hishammuddin said the difficulty of searching such a huge expanse of ocean made the operation in the southern corridor "much more challenging".

Officials believe that someone with detailed knowledge of both the Boeing 777 and commercial aviation navigation switched off two vital datalinks: the ACARS system, which relays maintenance data back to the ground, and the transponder, which enables the plane to be seen by civilian radar.

The source close to the investigation said that it was thought "highly probable that ACARS was switched off prior to the final verbal message" received for the cockpit.

That message, an informal "all right, good night" radioed to Malaysian air traffic controllers to acknowledge their handover of the plane to Vietnamese airspace, was believed to have been spoken by the co-pilot, the airline said earlier this week.

Investigators piecing together patchy data from military radar and satellites believe that minutes later the plane turned sharply west, re-crossing the Malay Peninsula and following an established commercial route towards India.

After that, ephemeral pings picked up by one commercial satellite suggest the aircraft flew on for at least six hours. The data from the satellite placed the plane somewhere in one of the two corridors when the final signal was sent at 8:11 a.m.

Hishammuddin said the latest in a series of reported possible sightings of the plane, this time over the Maldives, had been investigated by police in the Indian Ocean island chain and determined to be untrue.

FLIGHT SIMULATOR DATA DELETED

The methodical shutdown of the communications systems, together with the fact that the plane appeared to be following a planned course after turning back, have been interpreted as suggesting strongly that foul play, rather than some kind of technical failure, was behind the disappearance.

Police have searched the homes of the 53-year-old pilot Zaharie Ahmad Shah and co-pilot Fariq Abdul Hamid, 27. Among the items taken were a flight simulator Zaharie had built in his home.

Malaysia's police chief, Khalid Abu Bakar, said an examination of the flight simulator showed its data log had been cleared on Feb. 3. "The experts are looking at what are the logs that have been cleared," he told the news conference.

U.S. government sources said intelligence agencies had extensively analysed people on the flight but came up with no connections to terrorism or possible criminal motives.

A senior U.S. official said he was "not aware of any stones left unturned". China has said there is no evidence that Chinese passengers, who made up over two-thirds of those on board, were involved in a hijack or act of sabotage.

Australia is leading the search of the southern part of the southern corridor, with assistance from the U.S. Navy.

It has shrunk its search field based on satellite tracking data and analysis of weather and currents, but it still covers an area of 600,000 sq km (230,000 sq miles), roughly the size of Spain and Portugal.

The U.S. Navy said it had switched mainly to using P-8A Poseidon and P-3 Orion aircraft to search for the missing plane instead of ships and helicopters.

"The maritime patrol aircraft are much more suited for this type of operation," said Navy Lieutenant David Levy, who is on board the USS Blue Ridge. "...It's just a much more efficient way to search." (Additional reporting by A. Ananthalakshmi, Siva Govindasamy, Michael Martina and Niluksi Koswanage in Kuala Lumpur, Andrea Shalal-Esa and Mark Hosenball in Washington, Jane Wardell in Sydney, Peter Apps in London, Daniel Bosley in Male and Shihar Aneez in Colombo; Writing by Alex Richardson; Editing by Nick Macfie)