Australian authorities have said they are launching the underwater hunt for the black box from missing flight MH370.
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 (149 miles) 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".
The plane's black box recorders emit a ping that can be detected by equipment on board the ships - but the battery-powered devices stop transmitting about 30 days after a crash.
With the clock ticking down since MH370 went missing on March 8, Mr Houston acknowledged time is running out for search crews.
Mr Houston said: "The locater beacon will last about a month before it ceases its transmissions - so we're now getting pretty close to the time when it might expire."
Locating the data recorders and wreckage after the devices stop working is possible, but incredibly difficult.
The area the ships are searching was chosen based on hourly satellite pings the aircraft gave off after it vanished from radar on its route from Kuala Lumpur to Beijing with 239 people on board.
That information, combined with data on the estimated speed and performance of the aircraft, had led them to that specific part of the ocean, Mr Houston said.
Because the US Navy's pinger locator can pick up black box signals up to a depth of 6,100m (20,000ft), it should be able to hear the devices even if they are lying in the deepest part of the search zone - about 5,800m (19,000 ft) below the surface - if it gets within range of the black boxes.
But the task for search teams is hampered by the size of the search area and the fact the pinger locator must be dragged slowly through the water at just one to five knots, or one to six miles per hour.
Finding floating wreckage is key to narrowing the search area, as officials can then use data on ocean currents to try and backtrack to the spot where the Boeing 777 entered the water - and where the data recorders may be.
Despite weeks of fruitless searching, Mr Houston said he hadn't given up hope something would be found.
"I think there's still a great possibility of finding something on the surface," he said. "There's lots of things in aircraft that float."
The search area has shifted each day, as the investigative team continues to analyse available radar and satellite data while factoring in where any debris may have drifted due to ocean currents and weather.
Mr Houston said: "I think we've probably got to the end of the process of analysis. And my expectation is that we're into a situation where the data we've got is the data we've got and we'll proceed on the basis of that."
He said it was unlikely that any additional pinger locators would join the search any time soon as they are in limited supply.
Although Australia is coordinating the ocean search, the investigation into the plane's disappearance ultimately remains Malaysia's responsibility.
Australia, the US, Britain and China have all agreed to be "accredited representatives" of the investigation.
Four Australian investigators are in Kuala Lumpur to help with the investigation and ensure that information on the aircraft's likely flight path is fed back to search crews.
The two countries are still working out who will be in charge of the analysis of any wreckage and flight recorders that may be found, Mr Houston said.
On Thursday, the HMS Echo reported one alert as it searched for sonic transmissions from the missing plane's flight data recorder, but it was quickly discounted as a false alarm, the search coordination centre said.
False alerts can come from animals such as whales, or interference from shipping noise.