The search for missing Malaysia Airlines Flight MH370 in remote seas off Australia headed underwater on Friday, with a US Navy high tech "black box" locator deployed for the first time as the battery life of the cockpit data recorder dwindles.

Australian authorities said the so-called Towed Pinger Locator will be pulled behind navy ship HMAS Ocean Shield, searching a converging course on a 240 km (150 miles) track with British hydrographic survey ship HMS Echo.

"The area of highest probability as to where the aircraft might have entered the water is the area where the underwater search will commence," Retired Air Chief Marshal Angus Houston, the head of the Australian agency coordinating the operation, told reporters in Perth.

"On best advice the locator beacon will last about a month before it ceases its transmissions so we're now getting pretty close to the time when it might expire."

On Monday it will be 30 days since the jetliner lost communications and disappeared from civilian radar less than an hour into an overnight flight from Kuala Lumpur to Beijing on March 8.

The Boeing 777 was briefly picked up on military radar on the other side of Malaysia and analysis of subsequent hourly electronic "handshakes" exchanged with a satellite led investigators to conclude the plane crashed far off the west Australian coast hours later.

Sonar may help find the plane's black box voice and data recorders that are key to unlocking what happened on the flight. The black box is equipped with a locator beacon that transmits "pings" when underwater, but which only has an expected battery life of around 30 days.

Experts have warned the Towed Pinger Locator may be of little use unless investigators can get a much better idea of exactly where the plane went into water, because its limited range and the slow speed at which it must be pulled behind the ship mean it cannot cover large areas of ocean quickly.

Houston said the start of the underwater search in earnest did not override the need to keep searching for surface wreckage of the plane, as a find would be the most effective way to pinpoint a sub-sea hunt.

"This is a vast area, an area that's quite remote. We will continue the surface search for a good deal more time," he said.

"I think there's still a great possibility of finding something on the surface," he said. "There's lots of things in aircraft that float. In previous searches life jackets have appeared which can be connected to the aircraft that was lost."

HUGE SEARCH AREA

On Friday, up to 14 planes and nine ships were scouring the search area of about 223,000 sq km (86,000 sq miles) - roughly the size of the U.S. state of Minnesota - some 1,680 km (1,040 miles) west-north-west of Perth, he said.

Britain is also sending HMS Tireless, a Trafalgar-class nuclear submarine with sonar capabilities and a Malaysian frigate was due to arrive in the search area on Saturday.

Malaysian Prime Minister Najib Razak on Thursday joined his Australian counterpart Tony Abbott in a tour of RAAF Base Pearce, near Perth, where aircrews from seven countries have been flying dozens of missions deep into the southern Indian Ocean.

Malaysian authorities have faced heavy criticism, particularly from China, for mismanaging the search, now in its fourth fruitless week, and holding back information. Most of the 239 people on board the flight were Chinese.

"The world expects us to do our level best, and I'm very confident we will indeed show what we can do together as a group of nations; that we want to find answers, that we want to provide comfort to the families and we will not rest until answers are indeed found," Najib said.

Authorities have not ruled out mechanical problems as causing the disappearance, but say all the evidence suggests the plane was deliberately diverted from its scheduled route.

Malaysia's police chief said the investigation was focusing on the cabin crew and pilots, after clearing all 227 passengers of possible involvement in hijacking, sabotage or having personal or psychological problems that could have been connected to the disappearance. – Reuters