Missing plane MH370: Abbott says signal 'rapidly fading'
Signals in remote seas thought to be from the missing Malaysia Airlines flight MH370 are "rapidly fading" and finding the jet will be a "massive, massive task", Australia's PM says.
Tony Abbott said he was confident "pings" detected by search teams were from the aircraft's black boxes.
But no new signals have been confirmed in the search area since Tuesday.
"No one should underestimate the difficulties of the task still ahead of us," Mr Abbott warned.
Correspondents say Mr Abbott appeared to be qualifying his comments from Friday, in which he said he was "very confident" that signals heard by an Australian search ship were from the missing Boeing 777.
Speaking during a visit to China, Australia's leader said teams were hoping to track further signals in a section of the southern Indian Ocean before shifting the search operation to the seabed.
"Trying to locate anything 4,500 metres (15,000 feet) beneath the surface of the ocean, about a 1,000km (620 miles) from land is a massive, massive task," he said.
"Given that the signal from the black box is rapidly fading, what we are now doing is trying to get as many detections as we can so that we can narrow the search area down to as small an area as possible."
Mr Abbott said a submersible drone would be sent to conduct a sonar search of the seabed once search teams were confident with the area identified - but he refused to say when that might be.
After analysing satellite data, officials believe the plane with 239 people aboard flew off course for an unknown reason and went down in the southern Indian Ocean off the west coast of Australia.
Those leading the search fear that time is running out because the batteries that power the pings from the black box only last about a month, and that window has already passed.
Two sounds heard a week ago by the Australian ship Ocean Shield, towing the ping locator, were determined to be consistent with the signals emitted from the black boxes. Two more pings were detected in the same general area on Tuesday.
On Thursday an Australian aircraft picked up an audio signal in the same area as the four previous detections but officials now believe it is unlikely to be related to the black boxes
The underwater search zone is currently a 1,300 sq km (500 miles) patch of the seabed, about the size of Los Angeles.
The submersible drone, Bluefin 21, takes six times longer to cover the same area as the ping locator and it would take about six weeks to two months for it to search the current zone.
Complicating matters is the depth of the seabed in that area. The signals are emanating from 4,500m (15,000 ft) below the surface, which is the deepest the Bluefin can dive. The search coordination centre said it was considering options in case a deeper-diving sub was needed.
Malaysia Airlines Flight 370 was flying from Kuala Lumpur to Beijing on 8 March when it lost contact with air traffic controllers and vanished from radar.
Of the 239 people on board, 153 were Chinese. Many relatives have been angered by what they perceive to be the Malaysian authorities' early misguided response to the flight's disappearance.
The plane disappeared over the South China Sea, between Malaysia and Vietnam, but it was a week before the search was widened based on evidence taken from radar and satellite tracking.
Officials are still no clearer as to why the plane strayed so far off course.
The backgrounds of both passengers and crew have been scrutinised as officials consider hijacking, sabotage, pilot action or mechanical failure as possible causes.