Australian Prime Minister Tony Abbott has admitted it was "highly unlikely" that any surface wreckage will be found from Flight MH370 when he announced a more intensive underwater search.
Australia is coordinating the hunt for the missing Malaysia Airlines Boeing 777.
The plane disappeared on 8 March with 239 people on board and is believed to have crashed into the southern Indian Ocean.
Aircraft and ships have been scanning the surface of vast tracts of remote ocean off the western Australian coast for more than 40 days with no sign of wreckage from the plane.
"I am now required to say to you that it is highly unlikely at this stage that we will find any aircraft debris on the ocean surface," Mr Abbott told reporters in Canberra.
"By this stage, 52 days into the search, most material would have become water logged and sunk.
"With the distances involved, all of the aircraft are operating at close to the limit of sensible and safe operation."
Mr Abbott said the search would now enter a new phase that would involve undersea efforts being ramped up, with authorities searching the ocean floor over a much larger area.
"Essentially though what we are looking to do is conduct as thorough an undersea search as is humanly possible," Mr Abbott said.
"If necessary, of the entire probable impact zone, which is roughly 700 kilometres by 80 kilometres."
The search area for MH370 has been defined by analysis of satellite data.
It was boosted by several detections of transmissions believed to have come from the plane's black box recorders before their batteries died.
A submersible Bluefin-21 scouring a search zone centred around one of these transmissions has so far failed to yield any results despite searching almost the entire area.
Mr Abbott said the Bluefin-21 would continue its hunt, while Australia in consultation with the Malaysia government was willing to engage one or more commercial companies to undertake the extra work.
"While the search will be moving to a new phase in coming weeks, it certainly is not ending," the prime minister said.