TOKYO - The sputtering search for a missing Malaysian airliner will be expanded to include a much larger swath of the Indian Ocean floor, Australia's prime minister said Monday, signaling a daunting new phase in the bid to find the aircraft's wreckage.

The next stage in the hunt for Malaysia Airlines Flight MH370 will focus entirely on underwater exploration of the depths, forgoing the use of airplanes and vessels to spot debris on the surface. By now, Australian Prime Minister Tony Abbott said, any wreckage is likely to have become "waterlogged and sunk."

A broader underwater search - requiring unmanned miniature submarines trawling the depths at walking speed - casts doubt about the chances of ever unraveling a confounding aviation mystery. It also drastically increases the cost and expands the time frame.

The underwater search was previously focused on a small area that required two weeks to examine. The expanded area is about 22,000 square miles, about the size of West Virginia, and could require six to eight months to fully scour.

More than seven weeks into the search, the countries involved, including the United States, have been bearing their own costs. But Abbott said Monday that Australia will seek contributions from other nations while also engaging private companies, selected with help from the Malaysian government.

"We will do everything we humanly can, everything we reasonably can, to solve this mystery," Abbott said at a news conference in Canberra, the Australian capital.

No debris has yet been found from the airplane, which disappeared March 8 with 239 passengers and crew on board en route from Kuala Lumpur to Beijing. Its likely end-point - the Indian Ocean off the west coast of Australia - was determined only from analysis of signals the plane transmitted to a satellite while still airborne.