PERTH, Australia — With the Malaysian jetliner mystery now five weeks old, officials have narrowed the search zone for the missing plane and are "very confident" the underwater signals they have heard are from its black box, Australia's prime minister said Friday.
At the same time, however, those electronic signals are fading, Tony Abbott added.
On a visit to China, Abbott briefed President Xi Jinping on the search for Flight 370, which vanished March 8 while flying from Kuala Lumpur, Malaysia, to Beijing, carrying 239 people, most of them Chinese. Based on an analysis of satellite data, officials believe the Boeing 777 flew off-course for an unknown reason and went down in the southern Indian Ocean off the west coast of Australia.
Crews involved in the hunt have in recent days focused on a more-targeted area in the ocean for the source of the electronic signals, Abbott said.
"We have very much narrowed down the search area and we are very confident that the signals that we are detecting are from the black box on MH370," he told reporters in Shanghai, referring to the plane's flight data and cockpit recorders.
Search crews are racing against time because the batteries powering the recorders' locator beacons last only about a month — and more than a month has passed since the plane disappeared. Finding the devices after the batteries fail will be extremely difficult because the water in the area is 4,500 meters (15,000 feet) deep.
"We're getting into the stage where the signal from what we are very confident is the black box is starting to fade," he added. "We are hoping to get as much information as we can before the signal finally expires."
The Australian ship Ocean Shield is towing a U.S. Navy device that detects signals from the flight recorders. Two sounds heard Saturday were determined to be consistent with the signals emitted from the black boxes. Two more sounds were detected in the same general area Tuesday.
"We are confident that we know the position of the black box flight recorder to within some kilometers," Abbott said. "But confidence in the approximate position of the black box is not the same as recovering wreckage from almost 4½ kilometers beneath the sea or finally determining all that happened on that flight."
Abbott told the Chinese leader that the next steps will be a "very long, slow and painstaking process."
An Australian air force P-3 Orion, which has been dropping sonar buoys into the water near where the Ocean Shield picked up the sounds, detected another possible signal Thursday, but Angus Houston, who is coordinating the search, said in a statement that an initial assessment had determined it was not related to an aircraft black box.
The Ocean Shield towed its ping locator to try to find additional signals Friday, and the Orions were continuing their hunt, Houston said. The underwater search zone is currently a 1,300-square-kilometer (500-square-mile) patch of the seabed, about the size of the city of Los Angeles.
"It is vital to glean as much information as possible while the batteries on the underwater locator beacons may still be active," Houston said in a statement.
The searchers want to pinpoint the exact location of the source of the signals so they can send down a robotic submersible to look for wreckage. A decision to use the sub could be "some days away," Houston said.
The Bluefin 21 submersible takes six times longer to cover the same area as the ping locator being towed by the Ocean Shield — about six weeks to two months to canvass the current underwater zone.
Complicating matters is the depth of the seabed in that area. The signals are emanating from 4,500 meters (15,000 feet) below the surface, which is the deepest the Bluefin can dive. The search coordination center said it was considering options in case a deeper-diving sub is needed.
The surface area to be searched for floating debris had been narrowed to 46,713 square kilometers (18,036 square miles) of ocean extending from 2,300 kilometers (1,400 miles) northwest of Perth. Up to 15 planes and 13 ships were conducting the visual search Friday.
In Malaysia, police observed a minute of silence at a Buddhist temple to remember those aboard Flight 370.
Gelineau reported from Sydney. Associated Press writers Nick Perry in Perth, Rod McGuirk in Canberra, Australia, Eileen Ng in Kuala Lumpur, Malaysia, and Gillian Wong in Beijing contributed to this report.