PERTH, Australia, April 4 -- Two ships on Friday commenced a subsurface search for black box of missing Malaysia Airlines flight MH370 after no confirmed debris have been found from the jetliner nearly four weeks into a massive hunt.

Equipped with a U.S. towed pinger locator, Australian navy vessel Ocean Shield will be coupled with British warship HMS Echo to comb a single 240 kilometer 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 Echo is fitted with an array of sensors and sidescan sonar for surveying the ocean floor.

The black box contains a locator beacon which sends out a signal that can be picked up by search equipment and the beacon is designed with a battery life of about 30 days.

MH370 with 239 people on board vanished on March 8 on its way from Kuala Lumpur to Beijing. Despite extensive scouring of remote southern Indian Ocean by planes and ships off the coast of Perth, where the plane is believed to have been crashed, no trace has been found.

A total of 10 military planes, four civil jets and nine ships are involved in Friday's search covering a zone of about 217,000 square kilometers, some 1,700 kilometers northwest of Perth.

Australia's ship HMAS Perth has been tasked to join the mission and is expected to reach the target area in about four days, said Houston. Malaysian frigate KD Lekiu, which has departed naval base Stirling in Perth, is due to arrive in the search area on Saturday.

Australia, meanwhile, has accepted Malaysia's invitation to join the investigation into the disappearance of MH370 as a fully accredited member.

"Australia has agreed both to lead the search and as an accredited representative to provide support for the Malaysian investigations," said Houston, who briefed Malaysian Prime Minister Najib Razak on the Australia-led retrieval effort during his tour to Perth on Thursday. The U.S., Britain and China are also accredited members.

Australia and Malaysia are currently drawing up a comprehensive agreement regarding Australia's role in search and investigation, such as critical decision points, the handling of accident victims, custody of aircraft wreckage and the downloading of intonation from flight recorders that might be recovered, Houston said.

According to the Chicago Convention, Malaysia, the flight carrier, is responsible for the MH370 investigation.

An Australian team made up of four senior investigators is in Kuala Lumpur for MH370 related investigation and for ensuring that relevant investigation information is considered while mapping out search strategies, he added.