Australian authorities said on Tuesday they had begun searching 600,000 square kilometres (230,000 square miles) of the remote Indian Ocean for missing the Malaysia Airlines flight MH370, and conceded it was like looking for a needle in a haystack, since the territory is roughly the size of Spain and Portugal combined.
Strong currents and high seas are making the task more daunting, Australia's maritime safety agency said.
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 American and New Zealand aircraft working with Australia.
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.
"We are taking this search very seriously. If it went south this is AMSA's best estimate of where we should look," said Young, adding that the area would be reassessed every day based on water movement and weather.
Four Australian long-range P-3C Orion maritime surveillance aircraft are involved along with a New Zealand Orion and a United States P-8 Poseidon plane. Young said China had also requested to be involved and that was being considered.
"We owe it to the people on this ill-fated flight and their families to do what we can to solve this tragic mystery," Prime Minister Tony Abbott told parliament on Tuesday.
One sortie from Perth was expected to be flown on Tuesday and up to five on Wednesday.
Ships have also been alerted to keep watch, although the vastness of Indian Ocean means very few pass through.
Six Australians were on the commercial flight carrying 239 passengers and crew with the majority of those on board either Chinese or Malaysian.
In the northern hemisphere, a separate search area is along an arc stretching from Malaysia through northern Thailand, Myanmar and China to Kazakhstan.