* Australian PM says 'highly unlikely" floating debris will be found
* Search to enter a new phase, focused on larger search area
* Operation could last up to eight months
* New phase of search operation to cost Australia A$60 mln (Adds quotes, details of commercial search and background)
By Matt Siegel
SYDNEY, April 28 (Reuters) - The chance of finding floating debris from a missing Malaysia Airlines jetliner has become highly unlikely, and a new phase of the search will focus on a far larger area of the Indian Ocean floor, Australian Prime Minister Tony Abbott said on Monday.
The search effort for Malaysia Airlines Flight MH370, which vanished while flying from Kuala Lumpur to Beijing on March 8 with 239 people on board, has so far failed to turn up any trace of wreckage from the plane.
Given the amount of time that has elapsed, Abbott said that efforts would now shift away from the visual searches conducted by planes and ships and towards underwater equipment capable of scouring the ocean floor with sophisticated sensors.
He admitted, however, that it was possible nothing would ever be found of the jetliner.
"We will do everything we humanly can, everything we reasonably can, to solve this mystery," he told reporters in Canberra.
Malaysia, China, Japan, New Zealand, South Korea, Britain and the United States are assisting Australia in conducting the most expensive search in aviation history.
It remains unclear what caused the Boeing 777 to veer sharply off its course and disappear from radar as it prepared to cross into Vietnamese airspace.
Malaysian authorities have still not ruled out mechanical problems, but say evidence suggests it was deliberately diverted from its scheduled route.
Malaysia is under pressure to bring closure to the grieving families by finding wreckage to determine definitively what happened to the aircraft.
But the empty expanse of water northwest of the Australian city of Perth is one of the most remote places in the world and also one of the deepest, making the search complicated.
Authorities had been focusing on a 10 square km (6.2 square mile) stretch of seabed about 2,000 miles from Perth, after detecting what they suspected was a signal from the plane's black box recorder on April 4.
The U.S. Navy Bluefin-21 underwater drone searching the seabed has so far failed to turn up any sign of the plane.
"We are still baffled and disappointed that we haven't been able to find undersea wreckage based on those detections," Abbott told reporters.
Abbott said that the new search area, which spans 700 km by 80 km (435 miles by 40 miles), could take between 6-8 months to completely examine, at a cost to Australia of as much as A$60 million ($55.69 million).
The search operations have up until now been handled primarily as a military operation by the countries involved, but Abbott said that one or more commercial companies would be hired by Australia and Malaysia to handle the next phase.
Angus Houston, head of the Australian agency coordinating the search effort, offered a sobering assessment of the operation.
"We haven't found anything anywhere that has any connection to MH370," Houston said during the Abbott news conference. ($1 = 1.0774 Australian Dollars) (Reporting by Matt Siegel; Editing by Simon Cameron-Moore and Michael Perry)