Australian Prime Minister Tony Abbott said Monday that the hunt for missing flight MH370 entered a new phase with seabed search to be expanded to a much larger area.

"If necessary, of the entire probable impact zone which is roughly 700 kilometers by 80 kilometers," Abbott said at a press conference after U.S. submarine drone Bluefin-21 completed a sea floor search of a focused area in the Indian Ocean and found no contacts of interest.

The focused area was defined as a circle of 10 km radius around the spot where the second suspicious signal of aircraft black boxes was heard on April 8.

Abbott said it is highly unlikely to find any aircraft wreckage on the ocean surface at this stage, indicating the visual search currently conducted by airplanes and vessels may be called off.

"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. 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," he said.

"Therefore we are moving from the current phase to a phase which is focused on searching the ocean floor over a much larger area."

Fifty-two days have passed since the Malaysian Airlines flight disappeared en route from Kuala Lumpur to Beijing on March 8, but nothing related to the plane has been found.

Abbott said the new phase of search would be focused on the sea bed of the expanded area, which would involve commercial contractors specialized in deep ocean search and may take six to eight months if weather conditions permit.

Contractual equipments could be deployed in weeks and the Bluefin-21 would continue its mission in adjacent areas, he added.

"The Bluefin-21 submersible will continue in operation. What we are doing though is looking to an intensified underwater search involving different technology, in particular using specialized side scan sonar equipment towed behind ships to scan the seabed for evidence of aircraft wreckage."

In addition, an Australian aircraft, most likely a P-3C Orion, will be on standby at short notice in case possible wreckage is identified.

The new phase involving commercial contractors is expected to cost 60 million AU dollar (55.8 million U.S. dollars).

Acknowledging that it is possible to never find the ill-fated plane, Abbott vowed to continue the search as thoroughly as " humanely possible".

Despite the fruitless search over the past weeks, Abbott said he still has "considerable degree of confidence" that the signals detected by U.S.-supplied towed pinger locator came from aircraft black boxes.

He also confirmed that Australia would maintain the leadership of the search operation while the Malaysians have indicated that they want Australia to be involved in all subsequent investigations.