An Australian ship has been unable to re-locate the signals, or 'pings', first heard at the weekend in its search for the black box of flight MH370.
Search coordinators say they will continue to trawl the Indian Ocean for several days before they consider sending down a mini submarine to investigate. The Australian ship Ocean Shield picked up the pinging sound twice, on Saturday and Sunday, but has since been unable to hear it again.
Search coordinator Angus Houston and Australia's Defence Minister David Johnston both said re-locating the ping was the best-case scenario for the search.
Mr. Houston said "There have been no further contacts with any transmission and we need to continue that for several days right up to the point at which there's absolutely no doubt that the batteries will have expired. 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".
He said narrowing the search area first was critical.
He added "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".
"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".
It would provide a precise target for the Bluefin 21 - a mini submarine - to aim for in the next stage of the search. The plan is to send the mini sub down to the depths of the Indian Ocean and create a sonar map of the seabed.
It would highlight any debris on the ocean floor, which would then be further investigated using photographic equipment.
If all goes to plan, confirmation that plane wreckage had been discovered would come in the form of a photograph beamed back above the water's surface.
The transmission of the sound detected by the Australian ship is 33.2khz. This contrasts with the 37.5 kHz which is the usual transmission from a black box.
Australian officials say they have checked with the black box manufacturers and it is still regarded as a credible reading. The pressure at the base of the ocean can affect transmission signals.
