Search ships today failed to detect any more underwater pulses possibly from the missing Malaysian jet but will continue their efforts for "several days" before an unmanned submarine is sent down to scour the seabed for the plane's debris.
Searchers were racing against time to locate the crucial black box data recorders of the ill-fated aircraft as its batteries are at the end of their life.
"There have been no further contacts with any transmission and we need to continue (searching) for several days right up to the point at which there's absolutely no doubt that the batteries will have expired," Air Chief Marshal (retd) Angus Houston, the head of the Joint Agency Coordination Centre (JACC) which is leading the search, said.
If, till then, the US Navy listening equipment being towed behind Australia's Ocean Shield fails to pick up any signals, an unmanned submarine on board the ship will be deployed to scour the southern Indian Ocean seabed.
Houston said the towed pinger was still trying to re-locate the signals and will do so for "several days".
On deploying 'Blue Fin 21', an unmanned submarine, Houston said, "until we stop the pinger search we will not deploy the submersible."
He said Ocean Shield required to be allowed to continue its work trying to find another signal.
"If we can get more transmissions we can get a better fix on the ocean floor which will enable a much more narrowly focused visual search for wreckage," Houston said.
Investigators are hoping that the signals recorded by Ocean Shield over the weekend could be locater beacons from the Flight MH370 data recorders, but they are not sure yet.
However, buoyed by the hope that they are closing in, the size of the search area was reduced today, focusing on 77,580 square kilometre area in the Indian Ocean about 2,270 kilometres northwest of Perth. This is about a third of the size of the previous search zone.
Finding the black box is crucial to know what happened on March 8 before the Beijing-bound Malaysia Airlines Boeing 777 with 239 people, including five Indians, on board disappeared under mysterious circumstances.
Houston said the chances of finding something on the surface were "diminishing with time".
Referring to the beacons on aircraft black boxes which had a nominal 30-day operating period, Houston said, "there is a chance that the locater beacon is about to cease transmission or has ceased transmission."
Australian Defence Minister David Johnston said that the search could take more time than expected. He said planes and ships were "flat out" trying to progress the latest "lead".
"I want to confirm that we have at least several days of intense action ahead of us," he said, adding that "it's very challenging," he said.
