Search crews in the Indian Ocean have failed to pick up more of the faint underwater sounds that may have been from the missing Malaysian jetliner’s black boxes.

The signals first heard late on Saturday and early on Sunday had sparked hopes of a break­through in the search for flight MH370, but Angus Houston, the retired Australian air chief marshal leading the search far off western Australia, said listen­ing equipment on the Ocean Shield ship has picked up no trace of the sounds since then.

Finding the black boxes quickly is critical because their locator beacons have a battery life of only about a month – and yesterday marked exactly one month since the plane vanished.

A submarine will be deployed to try to chart out any debris on the sea floor

Once the beacons blink off, locating the black boxes in such deep water would be an immensely difficult, if not impossible, task.

“There have been no further contacts with any transmission and we need to continue (searching) for several days right up to the point at which there’s absolutely no doubt that the batteries will have expired,” Mr Houston said.

If, by that point, the US navy listening equipment being towed behind the Ocean Shield has failed to pick up any signals, a submarine will be deployed to try to chart out any debris on the sea floor.

If the sub maps out a debris field, the crew will replace its sonar system with a camera unit to photograph any wreckage.

Earlier, Australia’s acting prime minister, Warren Truss, had said the Bluefin 21 auto­nomous sub was to be launched yesterday, but a spokesman for Truss said later the conflicting information was a misunderstanding and Truss acknowledged the sub was not being used immediately.

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