(Adds Malaysia Airlines CEO quote)
* Search area roughly two-thirds size of United States
* Countries may be unwilling to share militarily sensitive radar data
* Electrical fire theories believed unlikely
* Background checks find no militant or criminal links
By Anshuman Daga and Tim Hepher
KUALA LUMPUR, March 19 (Reuters) - Investigations into the mystery of a missing Malaysian jet appeared to be at a deadlock on Wednesday, with no conclusive evidence of foul play and doubts whether nations would share military tracking data that could show where the plane may have headed.
Eleven days have passed since Malaysia Airlines Flight MH370 went missing, and 26 nations are struggling to search for the airliner over an area more than two-thirds the size of the continental United States.
Malaysian and U.S. officials believe the aircraft was deliberately diverted but an exhaustive background search of the 239 passengers and crew aboard has not yielded any possible motive or link to terrorism.
Malaysia's top official in charge of the unprecedented operation said it was vital to reduce the scale of the search and renewed appeals for sensitive military data from its neighbours that Malaysia believes may shed light on where the airliner flew.
"All the efforts must be used to actually narrow the corridors that we have announced - I think that is the best approach to do it. Otherwise we are in the realm of speculation again," Malaysia's Acting Transport Minister Hishammuddin Hussein told reporters late on Tuesday.
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 since the search field is growing," said Navy Lieutenant David Levy, who is on board the USS Blue Ridge, the U.S. Navy ship that is coordinating the search effort.
"It's just a much more efficient way to search," he said.
Flight MH370 vanished from civilian 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 for Beijing.
Investigators piecing together patchy data from military radar and satellites believe that someone turned off vital datalinks and turned west, re-crossing the Malay Peninsula and following a commercial route towards India.
After that, ephemeral pings picked up by one commercial satellite suggest the aircraft flew on for at least six hours, but investigators have very little idea whether it turned north or south, triggering a search expanding across two hemispheres.
In the latest of a series of possible sightings of the plane, police in the Indian Ocean island chain of the Maldives said they were investigating reports that people on one of its outer islands had seen a low-flying airplane there early on March 8. The police gave no further details.
Malaysia Airlines Chief Executive Ahmad Jauhari Yahya said the fact that aircraft continued to exchange electronic "handshakes" with the satellite weighed against theories that the pilots were battling some kind of technical problem.
"There's a possibility of malfunction, but the satellite communication system was still active until the last time there was data at 8:11 a.m.," he said. "There's some functionality in the system, except the reporting system was disabled."
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 terror attack.
Unless there is some kind of breakthrough, either in the form of new data or a sighting of the plane, the investigation appears to be drifting towards deadlock, sources said.
HUGE SEARCH AREA
Asked how important military tracking data would be to resolving the mystery, Hishammuddin said, "It is very important. But in the case of Malaysia, we have actually put aside national security, national interest to get to where we are today."
A senior diplomat in the region said military and government leaders were studying Malaysia's request, but there was no word so far on whether any data would be exchanged.
Malaysia says it will have to buy a new radar system after revealing what it knew of the path the airliner took after turning back across its territory.
"It looks like the ball is in (others') court now and they need to decide what sort of military and other data they are willing to share with us," a Malaysian government source said.
Analysts say it will be difficult to persuade others to do the same, especially if the result would be to reveal weakness in their own defences given the numerous maritime and territorial boundary disputes going on in the region.
"Information and intelligence exchange is very sensitive in this part of the world where there is a lot of distrust and sovereign issues," said Ian Storey, a senior fellow at Singapore's Institute of Southeast Asian Studies.
"Countries are unwilling to share sensitive intelligence because if reveals their military capabilities - or lack of capabilities."
The search covers a total area of 2.24 million nautical miles (7.68 million sq km), from central Asia to the southern Indian Ocean.
Because of its size, scale of human loss and sheer uncertainty over what happened, the missing airliner looks set to establish itself as one of the most baffling air transport incidents of all time.
A breakthrough is still possible, experts say. Wreckage could be found, but the more time elapsed since the aircraft's disappearance the more it will be scattered.
"It's a mystery and it may remain a mystery," says Elizabeth Quintalla, chief air power researcher at the Royal United Services Institute in London. (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 and Peter Apps in London; Editing by Raju Gopalakrishnan and Alex Richardson)