A senior U.S. official said he was "not aware of any stones left unturned".  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.  Diplomats and safety experts said the investigation is hampered by the reluctance of many countries to share military intelligence.  Asked how important such 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.  (Read more: Malaysian Prime Minister: Actions on board MH370 were deliberate)  "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."  (Read more: MH370 still without trace as search efforts double)  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.
Investigators probing the disappearance of a Malaysia Airlines jetliner with 239 people on board have discounted reports the plane may have been sighted over the Maldives.