SYDNEY - Australian and American authorities said Tuesday they had begun searching 600,000 square kilometres (230,000 square miles) of the remote Indian Ocean for missing Malaysia Airlines flight MH370, and conceded it was like looking for a needle in a haystack.
The Malaysian government has revealed it believes the jet was deliberately diverted and flew for several hours after leaving its scheduled flight path - either north towards Central Asia, or towards the southern Indian Ocean.
Authorities in Kuala Lumpur on Monday asked Canberra to take responsibility for the "southern vector" of the operation to locate the Boeing 777 that disappeared on March 8 en route to Beijing, with help from American and New Zealand aircraft.
The Australian Maritime Safety Authority has taken charge and emergency response general manager John Young said they were focusing on an area 3,000 kilometres south-west of Perth.
"AMSA has defined a possible search area with information available to us from a range of sources both nationally and internationally," he said.
"This search will be difficult. The sheer size of the search area poses a huge challenge - the search area is more than 600,0000 square kilometres.
"A needle in a haystack remains a good analogy," he said, adding that planes were looking for wreckage or other debris on the surface only and were not equipped to search underwater.
"It will take at least a few weeks to search the area thoroughly," said Young.
The search zone has been narrowed by the last known satellite and military radar data received from the plane, coupled with analysis of possible routes if it had flown south.
Long-range P-8 Poseidon and P-3 Orion surveillance aircraft from the US Navy's 7th Fleet are also involved in the search.
In Manila, 7th fleet commander Vice Admiral Robert Thomas described the combing nature of the search mission as "like mowing the grass", using the planes' high-altitude radars that can spot virtually anything floating on the sea.
"We will continue to cover the areas with airborne assets because that's really what the problem set calls for now," Thomas told reporters aboard the fleet flagship USS Blue Ridge, which was on port call in Manila.
"Right now, we do not have a time frame for the search window. We are just gonna keep at it until we're told to stop."
Fleet spokesman Commander William Marks said the surveillance planes had already covered more than 190,000 square kilometres, but emphasised the enormity of the task ahead.
"If you superimpose a map of the United States from the northern part of the Indian Ocean to the southern part of Australia, it is as if we are looking for a few people somewhere between New York and California and we don't know where (they are)," he said.
Apart from the US aircraft, four Australian long-range P-3C Orion maritime surveillance planes and a New Zealand Orion are involved. Young said China had also requested to be involved and that was being considered.
Ships have also been alerted to keep watch, although the vastness of Indian Ocean means very few pass through.
A guided missile destroyer from the US Navy's 7th fleet had been involved in the hunt earlier in the Andaman Sea, but the Pentagon said on Monday it would be withdrawn because the area to be searched was too big.
Six Australians were on the commercial flight carrying 239 passengers and crew with the majority of those on board either Chinese or Malaysian.