Underwater search for flight MH370 commences off coast of Perth

A subsurface hunt for missing Malaysia Airline flight MH370 started Friday as searchers are racing against time to locate its data recorders known as black box before the batteries run out of power.[Special coverage]

Australia's navy vessel Ocean Shield equipped with a U.S. towed pinger locator and British oceanographic vessel HMS Echo with similar capabilities began scouring along a single 240- kilometer underwater track converging on each other, Angus Houston, chief of the Joint Agency Coordination Center (JACC) leading the search, told a press conference in Perth in Western Australia.

The hi-tech towed pinger locater is capable of picking up emissions from the black box pinger up to 6,100 meter under sea surface while the Echo is fitted with an array of sensors and sidescan sonar for surveying the ocean floor.

The black box contains a pinger with a battery life of about 30 days after a crash.

The MH370 with 239 people on board is believed to have crashed on March 8 in the southern Indian Ocean west of Perth, although no confirmed debris has been found from the plane.

The underwater search is normally in an area where the plane most likely entered the water, said Houston.

But in this particular case, the last known position was a long way from where the aircraft appeared to have gone, he added.

A total of 10 military planes, four civil jets and nine ships on Friday searched a zone of about 217,000 square kilometers, some 1,700 kilometers northwest of Perth as determined by the Australian Maritime Safety Authority (AMSA).

The search area is being constantly adjusted based on continuing groundbreaking and multi-disciplinary technical analysis of satellite communications and aircraft performance, passed from the international air crash investigative team, said Houston, the retired air chief marshal.

Describing the mission as the most difficult search in human history, Australian Prime Minister Tony Abbot has vowed to "throw everything at it."

Seven countries -- Australia, the U.S., China, New Zealand, Japan, South Korea and Malaysia are assisting in the huge search mission which has dragged on for nearly four weeks.