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Ships searching for underwater signals from Flight MH370 have heard no more "pings" and will spend several more days trying to pinpoint a crash site before launching a mini-sub to scour the ocean floor, authorities say.
A month to the day since the Boeing 777 vanished with 239 people on board, time is running out to detect further signals as the batteries in beacons on the jet's black box data recorders reach their expiry date.
Transmissions picked up by Australia's Ocean Shield ship consistent with those from aircraft black boxes had raised hopes that a robotic submersible would soon be sent down to look for debris.
But search chief Angus Houston clarified Tuesday that while the pings were an exciting development, further transmissions were needed before scouring the ocean floor.
"We need to continue that (search) for several days to the point at which there is absolutely no doubt that the pinger batteries will have expired," Houston said.
"Until we stop the pinger search we will not deploy the submersible."
He said that no further transmissions had been detected in the remote search area off western Australia which could help pinpoint where the jet carrying 239 people might have crashed.
The plane went missing en route from Kuala Lumpur to Beijing on March 8 and the search is now focusing on a 600-kilometre (370-mile) arc of the remote southern Indian Ocean.
Houston had indicated that the time was nearing for the Ocean Shield, which is criss-crossing the area to try to home in on the signals again, to launch the US-made autonomous underwater vessel Bluefin-21.
But he put a more definitive timeframe on it Tuesday, saying the five-metre (16-feet) long submersible sonar device would not be put into the water unless more transmissions were detected.
"If we go down there now and do the visual search it will take many, many, many days because it's very slow, very painstaking work to scour the ocean floor," he said.
Families of MH370 passengers in Beijing marked the one-month anniversary of the plane's disappearance with a tearful candlelit vigil Tuesday, desperately trying to comfort each other as their agonising wait for news continues.
About two-thirds of the 239 people on board were Chinese.
- Taking it to the limit -
The 4.5-kilometre (nearly three-mile) depth of the ocean floor is the absolute operating limit for a Bluefin-21, which is designed for deep sea surveying and can carry video cameras.
Ocean Shield, which picked up two series of pulses lasting two hours and 20 minutes and then 13 minutes, is operating at the northern end of the defined search area.
China's Haixun 01, which has also reported some acoustic sounds, and Britain's HMS Echo are working the southern end.
The signals are being investigated as the clock ticks past the 30-day lifespan of the emergency beacons on the two data recorders from the Malaysia Airlines jet.
Houston said the Ocean Shield transmissions were the most promising, adding that the advice from black box manufacturers was that the frequency of the signals could have changed.
"There's a change with the pressure on the ocean floor and the age of the particular batteries... you get changes in the transmission level," he said.
"Some of the false leads that we've had have been transmissions from the ship that was actually searching."
- Encouraging signs -
While 14 ships were involved in the search, it was not possible to crowd the area due to the need for silence, he said.
"We can't have too many ships in the area because when you're dealing with these transmissions you need utter silence," he said.
Commander William Marks of the US Seventh Fleet has said one of the signals picked up strengthened for a time, then weakened, indicating crews were near its source.
"That is encouraging because that is what you would expect if you are indeed moving toward the black box -- that it should get stronger and as you move away from it, it should get weaker," he told AFP.
Up to 11 military planes and three civilian planes were also Tuesday set to take part in the unprecedented search 2,268 kilometres northwest of Perth, the Joint Agency Coordination Centre (JACC) said.
Australian Defence Minister David Johnston said the searchers were working on the "positive lead" of the acoustic noises to deliver up something more tangible.
"I want to confirm that we have at least several days of intense action ahead of us," he said, adding that the weather was expected to be reasonable while the two pingers were still active.
"We are totally focused on assisting our Malaysian friends in identifying the location of this aircraft and bringing some closure to the loved ones of the passengers and crew onboard," he said.
Johnston said the search for debris floating on the water's surface was still going on.
If deployed, the Bluefin-21 could detect unusual formations on the ocean floor. It would then be brought back to the surface to fit cameras before returning to the seabed.
"What we're after is wreckage, a debris field as people would say," Houston said.