An underwater search has started in the Indian Ocean for the black box from the missing Malaysian jet.
The Australian and Royal Navy are using technology developed in the US to try to locate the 'pings' from the data recorder.
Flight MH 370 vanished a month ago.
Australian aviation expert Neil Hansford explains the significance of the search.
Air Chief Marshal Angus Houston told a news conference: "The Australian Navy and the Royal Navy have today commenced a sub-surface search for emissions from the black box pinger from Malaysia Airlines flight MH370.
"Using the towed pinger from the US Navy on Australian defence vessel Ocean Shield and a similar capability on HMS Echo, the two ships will search a single 240km track converging on each other."
It comes after Malaysia's opposition leader accused the government of deliberately concealing information about missing flight MH370.
Anwar Ibrahim, who personally knew the pilot of the Malaysia Airlines Boeing 777, called for an international committee to take over the Malaysian-led operation, saying "the integrity of the whole nation is at stake".
He indicated it was even possible that there was "complicity by authorities on the ground" in what happened to the plane and the 239 people on board.
Radar fail
In an interview with The Daily Telegraph, he said it was "not only unacceptable but not possible, not feasible" that the plane had not been sighted by the Marconi radar system immediately after it changed course.
He claimed the radar would have instantly detected the jet as it travelled east to west across "at least four" Malaysian provinces.
Mr Anwar told the newspaper it was "baffling" that the country’s air force had "remained silent", and suggested that it "should take three minutes under standard operating procedure for the air force planes to go. And there was no response".
Australian PM Tony Abbott said his country was "throwing everything at it" to find the plane, which disappeared on its way from Kuala Lumpur to Beijing on March 8.
No trace of the jet has been found almost four weeks after it vanished.
