The signals first heard late Saturday and early Sunday had sparked hopes of a breakthrough in the search for Flight 370, but Angus Houston, the retired Australian air chief marshal leading the search far off western Australia, said listening equipment on the Ocean Shield ship has picked up no trace of the sounds since then.
Finding the black boxes quickly is critical, because their locator beacons have a battery life of only about a month - and Tuesday marks exactly one month since the plane vanished. Once the beacons blink off, locating the black boxes in such deep water would be an immensely difficult, if not impossible, task.
"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," Houston said.
If, by that point, the U.S. Navy listening equipment being towed behind the Ocean Shield has failed to pick up any signals, a submarine will be deployed to try to chart out any debris on the seafloor. If the sub maps out a debris field, the crew will replace its sonar system with a camera unit to photograph any wreckage.
Earlier, Australia's acting prime minister, Warren Truss, had said the Bluefin 21 autonomous sub would be launched on Tuesday, but a spokesman for Truss said later the conflicting information was a misunderstanding, and Truss acknowledged the sub was not being used immediately.
The two distinct sounds the towed pinger locator detected are consistent with the pings from an aircraft's black boxes, Houston said.
Defense Minister David Johnston called the sounds the most positive lead and said it was being pursued vigorously. Still, officials warned it could take days to determine whether the sounds were connected to the plane that vanished March 8 on a flight from Kuala Lumpur, Malaysia, to Beijing with 239 on board.
"This is an herculean task - it's over a very, very wide area, the water is extremely deep," Johnston said. "We have at least several days of intense action ahead of us."
Houston said finding the sound again was critical to narrowing the search area before the sub can be used. If the vehicle went down now with the sparse data collected so far, it would take "many, many days" for it to cover all the places the pings might have come from.
"It's literally crawling at the bottom of the ocean so it's going to take a long, long time," Houston said.
He also warned of past false leads - such as ships detecting their own signals. Because of that, other ships are being kept away, so as not to add unwanted noise.
"We're very hopeful we will find further evidence that will confirm the aircraft is in that location," Houston said. "There's still a little bit of doubt there, but I'm a lot more optimistic than I was one week ago."
The black boxes containing the flight data and cockpit voice recorders are key to learning why the plane lost communications and veered so far off-course.
"Everyone's anxious about the life of the batteries on the black box flight recorders," said Truss, who is acting prime minister while Tony Abbott is overseas. "Sometimes they go on for many, many weeks longer than they're mandated to operate for - we hope that'll be the case in this instance. But clearly there is an aura of urgency about the investigation."
The first sound picked up by the equipment on board the Ocean Shield lasted two hours and 20 minutes before it was lost, Houston said. The ship then turned around and picked up a signal again - this time recording two distinct "pinger returns" that lasted 13 minutes. That would be consistent with transmissions from both the flight data recorder and the cockpit voice recorder.
The black boxes normally emit a frequency of 37.5 kilohertz, and the signals picked up by the Ocean Shield were both 33.3 kilohertz, U.S. Navy Capt. Mark Matthews said.
Houston said the frequency heard was considered "quite credible" by the manufacturer, and noted that the frequency from the Air France jet that crashed several years ago was 34 kilohertz. The age of the batteries and the water pressure in the deep ocean can affect the transmission level, he said.
The Ocean Shield is dragging a pinger locator at a depth of 3 kilometers (1.9 miles). It is designed to detect signals at a range of 1.8 kilometers (1.12 miles), meaning it would need to be almost on top of the recorders to detect them if they were on the ocean floor, which is about 4.5 kilometers (2.8 miles) deep.
The surface search for any plane debris also continued Tuesday. Up to 14 planes and as many ships were focusing on a single search area covering 77, 580 square kilometers (29,954 square miles) of ocean, said the Joint Agency Coordination Center, which is overseeing the operation.
Associated Press writers Kristen Gelineau in Sydney and Rod McGuirk in Canberra, Australia, contributed to this report.