An Australian ship which picked up possible "pings" from the black box recorders of a missing Malaysia Airlines jetliner has been unable to detect any further signals and time is running out to narrow the massive search, officials have said.
Angus Houston, head of the Australian agency coordinating the search for Malaysia Airlines Flight MH370, said the month-long hunt in the Indian Ocean was at a critical stage given the batteries in the black box beacons had already reached the end of their 30-day expected life.
"If we don't get any further transmissions, we have a reasonably large search area of the bottom of the ocean to prosecute and that will take a long, long time. It's very slow, painstaking work," said Houston.
A US Navy "towed pinger locator" onboard Australia's Ocean Shield picked up two "ping" signal detections over the weekend - the first for more than two hours and the second for about 13 minutes.
It is a large area for a small submersible that has a very narrow field of search, and of course, it is literally crawling along the bottom of the ocean
Houston said the signals sounded very much like black box beacons and represented the best lead in the search yet, but efforts to pick up the pings again had so far been unsuccessful.
The black boxes record cockpit data and may provide answers about what happened to the plane, which was carrying 227 passengers and 12 crew when it vanished on March 8 and flew thousands of kilometres off its Kuala Lumpur-to-Beijing route.
Authorities have not ruled out mechanical problems as a cause of the plane's disappearance but say evidence, including loss of communications, suggests it was deliberately diverted.
Analysis of satellite data led investigators to conclude the Boeing 777 came down in an area some 1,680km northwest of Perth, where possible pings were picked up and the search is now focused.
An autonomous underwater vehicle (AUV) named Bluefin-21 is onboard the Ocean Shield and could be sent to look for wreckage on the sea floor, but narrowing the search zone first was critical, Houston said.
"It is a large area for a small submersible that has a very narrow field of search, and of course, it is literally crawling along the bottom of the ocean," he said.
"That's why its so important to get another transmission and we need to continue until there's absolutely no chance the device is still transmitting."
Some 133 missions have been completed so far in the multinational aerial hunt for debris in the southern Indian Ocean but have only turned up fishing gear and other detritus.