PERTH, Australia (AP) -- An Australian aircraft hunting for the missing Malaysian jet picked up a new possible underwater signal on Thursday in the same area search crews previously detected sounds that were consistent with an aircraft's black boxes.
The Australian navy P-3 Orion, which has been dropping sound-locating buoys into the water near where the original sounds were heard, picked up a "possible signal" that may be from a man-made source, said Angus Houston, who is coordinating the search off Australia's west coast.
"The acoustic data will require further analysis overnight," Houston said in a statement.
If confirmed, this would be the fifth underwater signal detected in the hunt for Flight 370, which vanished on March 8 while flying from Kuala Lumpur, Malaysia, to Beijing, with 239 people aboard.
On Tuesday, the Australian vessel Ocean Shield picked up two underwater sounds, and an analysis of two other sounds detected in the same general area on Saturday showed they were consistent with a plane's flight recorders, or "black boxes."
The Australian navy has been dropping buoys from planes in a pattern near where the Ocean Shield's signals were heard.
Royal Australian Navy Commodore Peter Leavy said each buoy is dangling a hydrophone listening device about 300 meters (1,000 feet) below the surface. The hope, he said, is that the buoys will help better pinpoint the signals, along with the Ocean Shield, which is slowly dragging a U.S. navy pinger locator through the water.
Zeroing in on the source of the sounds is critical to narrowing down the underwater search zone, which is currently a 1,300 square kilometer (500 square mile) patch of the ocean floor. Once the exact location is pinpointed, crews can send an unmanned submarine into the ocean's depths to create a sonar map of a potential debris field on the seabed.
The Bluefin 21 sub takes six times longer to cover the same area as the pinger locator, and it would take the vehicle about six weeks to two months to canvass the underwater search zone, which is about the size of Los Angeles. That's why the acoustic equipment is still being used to hone in on a more precise location, U.S. Navy Capt. Mark Matthews said.
Meanwhile, a hunt for debris on the ocean surface intensified Thursday, with the search zone narrowed to its smallest size yet - 57,900 square kilometers (22,300 square miles), or about one-quarter the size it was a few days ago. Fourteen planes and 13 ships were taking part in the hunt for floating debris, about 2,300 kilometers (1,400 miles) northwest of Perth.
A "large number of objects" had been spotted by crews combing the area for floating debris on Wednesday, but the few that had been retrieved by search vessels were not believed to be related to the missing plane, the search coordination center said.
Crews hunting for debris on the surface have already looked in the area they were crisscrossing on Thursday, but were moving in tighter patterns, now that the search zone has been narrowed to about a quarter the size it was a few days ago, Houston said.
Houston has expressed optimism about the sounds detected earlier in the week, saying on Wednesday that he was hopeful crews would find the aircraft - or what's left of it - in the "not-too-distant future."
Finding the flight data and cockpit voice recorders soon is important because their locator beacons have a battery life of about a month, and Tuesday marked one month since Flight 370 disappeared. The plane veered off-course for an unknown reason, with officials saying that satellite data indicates it went down in the southern Indian Ocean. The black boxes could help solve that mystery.
But if the batteries fail before the recorders are located, finding them in such deep water - about 4,500 meters, or 15,000 feet - would be difficult, if not impossible.
A data analysis of the signals heard Saturday indicated they were distinct, man-made and pulsed consistently - indicating they are coming from an aircraft's black box, Houston said.
An Australian government briefing document circulated among international agencies involved in the search on Thursday said it was likely that the acoustic pingers would continue to transmit at decreasing strength for up to 10 more days, depending on conditions.
Once there is no hope left of the Ocean Shield's equipment picking up any more sounds, the Bluefin sub will be deployed.
Complicating matters, however, is the depth of the seafloor in the search area. The pings detected earlier are emanating from 4,500 meters below the surface - which is the deepest the Bluefin can dive.
"It'll be pretty close to its operating limit. It's got a safety margin of error and if they think it's warranted, then they push it a little bit," said Stefan Williams,
a professor of marine robotics at Sydney University.
The search coordination center did not immediately respond to a question about its contingency plans should the black box be too deep for the sub to reach. But Williams suspects if that happens, the search will be delayed while an underwater vehicle rated to 6,000 meters (19,700 feet) was dismantled and air freighted from Europe, the U.S. or perhaps Japan.
Williams said colleagues at the Woods Hole Oceanographic Institution in Massachusetts had autonomous and remotely operated underwater vehicles that will dive to 11 kilometers (36,100 feet), although they might not be equipped for such a search.
Underwater vessels rated to 6,500 meters (21,300 feet) could search the sea bed of more than 90 percent of the world's oceans, Williams said.
"There's not that much of it deeper than six and a half kilometers," he said.
Williams said it was unlikely that the wreck had fallen into the narrow Diamantina trench, which is about 5,800 meters (19,000 feet) deep, since sounds emanating from that depth would probably not have been detected by the pinger locator.
Gelineau reported from Sydney. Associated Press Writer Rod McGuirk in Canberra, Australia, contributed to this report.