The intensive air search for wreckage from Flight MH370 officially ended Wednesday as the hunt was drastically scaled back, with ships also leaving the Indian Ocean area where the plane is believed to have crashed.

Australian authorities said the focus would move "over the coming weeks" to an intensified undersea search in the quest to find out what happened to the Malaysia Airlines flight that disappeared on March 8 with 239 people aboard.

Eight nations have been involved in the unprecedented Indian Ocean hunt -- Australia, New Zealand, Malaysia, Japan, South Korea, the United States, Britain and China -- with more than 300 sorties flown across a vast expanse of water in the search for debris.

But with nothing to show for their efforts to scan more than 4.5 million square kilometres (1.7 million square miles) from the air since March 18, the planes have been stood down.

"Most of the aircraft will have left by the end of today," a spokesman for the Australian-led Joint Agency Coordination Centre told AFP, although an Australian P-3 Orion would remain on standby in Perth.

The United States, Japan, New Zealand and Malaysia all confirmed that their aircraft were returning to base.

Beijing's foreign ministry spokesman Qin Gang declined to specify whether China's aircraft were being withdrawn. The majority of the passengers on board the missing jet were Chinese.

"I repeatedly said that, in the next phase, the Chinese side will continue to actively support and take an active part in the search operations," he told reporters at a regular briefing.

"We will stay in close communication and coordination with all relevant parties."

As many as 14 ships from Australia, China and Britain were involved in scanning the ocean surface for debris or black box signals but many of these are also pulling out.

"Some need to head back to port and refuel and give the crew a rest, others will go back to doing what they were doing for their respective nations before they joined the search," the spokesman said.

"In essence, the surface search has been scaled back. We will keep a few vessels out there and others on standby, but the large-scale air and sea search has ended."

- Underwater search 'could take eight months' -

On Monday Australian Prime Minister Tony Abbott announced the focus would shift to an expanded underwater search across a huge swathe of seabed where the plane might have crashed, admitting it was now "highly unlikely" that any surface wreckage would be found.

A US Navy submersible Bluefin-21 has been scouring a 314-square kilometre zone centred around one of the transmissions believed to have come from the plane's black box flight recorders before their batteries died.

But it too has failed to find anything, with poor weather hampering efforts on Wednesday.

Abbott said an area of up to 56,000 square kilometres of the ocean floor would be scoured in the new search -- with the Bluefin remaining in operation along with other technology, possibly a specialised side-scan sonar.

He estimated it would take six to eight months.

Despite the failure to find wreckage, authorities insist they are looking in the right area and on Wednesday dismissed claims by a marine exploration company that material found in the Bay of Bengal could be from the missing flight.

Adelaide-based GeoResonance was quoted in Malaysian and Australian media as saying it had detected possible debris from a plane 5,000 kilometres (3,100 miles) from the current search zone.

But the Joint Agency Coordination Centre played down any link.

"The Australian-led search is relying on information from satellite and other data to determine the missing aircraft's location. The location specified by the GeoResonance report is not within the search arc derived from this data," a spokesman said.

"The joint international team is satisfied that the final resting place of the missing aircraft is in the southerly portion of the search arc."

GeoResonance, which specialises in geophysical surveys to find oil and gas and groundwater, said its research using images from satellites and aircraft had identified elements on the ocean floor consistent with material from a plane.

"We identified chemical elements and materials that make up a Boeing 777... These are aluminium, titanium, copper, steel alloys and other materials," company representative Pavel Kursa told Australia's Channel Seven.

Another company official, David Pope, told the broadcaster: "We're not trying to say that it definitely is MH370. However, it is a lead we feel should be followed up."