Pings heard by search crews in the southern Indian Ocean over the weekend, which have been called the most promising clues yet in the search for missing Malaysia Airlines Flight MH370, have fallen silent, adding to fears that the batteries on the plane’s flight recorders, also called black boxes, may have run out of power.

"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," Angus Houston, head of the search's Joint Agency Co-ordination Center, said on Tuesday, according to Associated Press.


Houston added that submarine vehicles such as the Bluefin 21, which reports earlier said would be deployed to scour the ocean bed, would remain on standby until further pings are heard or it can be concluded that the black box batteries have died out. 


Earlier in the day, Australia's acting Prime Minister Warren Truss had said search crews would launch the Bluefin 21 autonomous sub on Tuesday, AP reported, adding that Truss acknowledged the sub was not being used immediately.


The missing plane disappeared on March 8, with 239 people on board, on a flight from Kuala Lumpur to Beijing, triggering an unprecedented international search operation, which entered its 32nd day on Tuesday, and is expected to potentially cost hundreds of millions of dollars.


A report from Reuters, citing estimates by defense analysts and the Pentagon, said that the search operation, which includes planes, ships and satellites contributed by more than two dozen countries, has so far cost at least $44 million, and could become the most expensive search in aviation history.