The battery on the Malaysia Airlines plane’s black box is expected to expire some time after April 7, 2014

The international search for the missing Malaysian Airlines flight MH370 moved underwater on Friday, the search coordinator announced.

An Australian navy ship and a British navy ship are engaged in a joint underwater search operation towing sensitive sonic detectors listening for the signal of the plane’s black box.

Joint Agency Coordination Centre chief Angus Houston told reporters in Perth the ships had been deployed 240 km apart in a gradually converging course as they sweep the ocean floor for the ping emitted by the black box.

The Australian ship Ocean Shield is towing a U.S. Navy signal detector at just 5 kmph, while the British Royal Navy hydrographic ship HMS Echo has its own sensitive underwater signal detection equipment.

The battery on the black box is expected to expire some time after April 7, but could last until the middle of April.

Mr. Houston said two submarines were now also engaged in the search.

Four civil jets joined 10 military planes on Friday in the search for the plane, which went missing nearly four weeks ago on its way from Kuala Lumpur to Beijing.

Australian volunteers aboard three of the civil aircraft will work as air observers, while the fourth plane will act as a communications relay, JACC said.

A Malaysian frigate will arrive in the zone on Saturday, and an Australian frigate will join the search in four days, Mr. Houston said.

JACC said British navy ship HMS Echo picked up an alert on Thursday while on the way to the search, but it did not come from the plane’s black box, the Australian Broadcasting Corporation reported.

Noises from whales or ships could trigger an alert on the ship’s sensitive sonic receptors, JACC said.

The multinational operation in the Indian Ocean is now in its 18th day, with the 2,17,000-square-kilometre search zone shifting slightly north of previous unsuccessful searches.

The search is intensifying as time to find the black box flight recorder is running out.

The Australian Transport Safety Bureau said it is continuing to work on narrowing down the area where the plane could have hit the water using data supplied by an air crash investigation team with analysts from Malaysia, the United States, Britain, China and Australia.

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