Australia's prime minister Monday announced an expanded search across a huge swathe of seabed where Flight MH370 might have crashed seven weeks ago, admitting it is now "highly unlikely" any surface wreckage will be found.

A massive hunt for the Malaysia Airlines Boeing 777 in the southern Indian Ocean has so far yielded nothing either on the surface or below, baffling authorities who are struggling to explain the loss of the aircraft.

"I regret to say that thus far, none of our efforts in the air, on the surface, or undersea have found any wreckage," Tony Abbott said.

He added it was now "highly unlikely" authorities would find surface debris, noting that an area of more than 4.5 million square kilometres (1.5 million square miles) had been scanned from the air.

"By this stage, 52 days into the search, most material would have become waterlogged and sunk."

Flight MH370 disappeared on March 8 carrying 239 people and is believed to have crashed in the southern Indian Ocean off west Australia after mysteriously diverting from its Kuala Lumpur to Beijing journey.

Abbott said the search would now enter a new phase of intensified undersea efforts, with authorities scouring the ocean floor over an area of about 56,000 square kilometres.

"If necessary, of the entire probable impact zone which is roughly 700 kilometres by 80 kilometres," he said when asked about the extent of the search area.

The search zone has been defined by analysis of satellite data. Searchers' hopes were boosted by several detections of transmissions believed to have come from the plane's black box flight recorders before their batteries died.

But the US Navy submersible Bluefin-21, scouring a 400-square kilometre zone centred around one of these transmissions, has failed to yield results.

Abbott announced a hugely expanded underwater search involving different technology, possibly a specialised side-scan sonar.

- Satellite data questioned -

The announcement could raise questions over the satellite data Malaysia has used to settle on the suspected Indian Ocean crash zone, said Ravikumar Madavaram, consultant for aerospace & defense at Frost & Sullivan in Malaysia.

Families of relatives, frustrated by the lack of progress, have also questioned whether the data needs to be reviewed.

"We have finished the entire Bluefin search area, so we could be searching in the wrong window. We should go back and double-check our satellite data and assumptions and expand possibilities of where it could be," said Ravikumar.

He added: "it's going to be a costly affair" after Abbott's indication of an expanded seabed search.

Abbott said the Australian government, in consultation with Malaysian authorities, was willing to engage one or more commercial companies to undertake the work.

He estimated it would cost Aus$60 million (US$55.8 million) and take six to eight months.

So far, the eight nations involved in the Indian Ocean search -- Australia, New Zealand, Malaysia, Japan, South Korea, the United States, Britain and China -- have borne their own costs, but Abbott said Canberra would ask for help in funding the next stage.

Australia is working closely with Malaysia and China, whose citizens made up most of those onboard the flight, and Beijing said it would actively support the next phase.

In the meantime the Bluefin-21 and several ships would maintain the search and a team of experts in Kuala Lumpur from Malaysia, China, Australia, Britain and the US would work on refining where to look.

Abbott said authorities still had "a considerable degree of confidence" that the signals picked up were from the black box and were determined to pursue that lead.

The anguish of families has repeatedly spilled over into anger against the Malaysian government and airline for their handling of the unprecedented event -- but the lack of wreckage had them questioning the search on Monday.

Steven Wang, whose mother was on the plane, said the expansion plan suggests the search was not looking in the right place.

"But they just search the whole ocean, and it is such a big ocean, and you cannot search everywhere. It will take years," he said.

Sarah Bajc, whose American partner Philip Wood was on the flight, questioned why Australia continued to pour assets into searching in one area.

"Why not just open up the raw (satellite) data for independent review and let's see what happens?" she asked in an email to AFP.

Selamat Omar of Malaysia, whose 29-year-old son Khairul Amri Selamat was on the plane, said there were still many unknowns.

"With no sign of a crash, I have to think that my son could still be alive," he said.