Authorities are confident that a series of underwater signals in the Indian Ocean are coming from a missing Malaysia Airlines plane, Australia’s prime minister said Friday, while warning that those signals are beginning to fade.

Tony Abbott told reporters in Shanghai that searchers have narrowed the area where they hope to locate the plane’s wreckage on the ocean floor by listening for pings from emergency locator beacons built into the plane’s flight data and cockpit voice recorders.

Audio transmissions were picked up by the Australian navy vessel Ocean Shield in four locations April 5 and Tuesday by a device called a towed pinger locator supplied by the U.S. Navy and dangled deep in the sea.

But no further sounds have been heard since Tuesday, and efforts to triangulate those pings to a more precise location for the plane’s wreckage are racing against time, as the batteries powering those beacons have surpassed their advertised 30-day shelf life.

“We are confident that we know the position of the black box flight recorder to within some kilometers, but confidence in the approximate position of the black box is not the same as recovering wreckage from almost 41 / 2 kilometers [about three miles] beneath the sea or finally determining all that happened on that flight,” Abbott said.

A fish and a robot search for Flight 370

Abbott said that some of the audio signals have been tracked over “quite a long period of time,” helping to reduce the search area.

“Nevertheless, we’re getting to the stage where the signal from what we are very confident is the black box is starting to fade,” he said. “We are hoping to get as much information as we can before the signal finally expires.”

A fifth acoustic signal was picked up by an Australian aircraft Thursday, but retired Air Chief Marshal Angus Houston, who is in charge of the search, said detailed analysis suggested that it was “unlikely to be related” to the missing plane.

“Further analysis continues to be undertaken by Australian Joint Acoustic Analysis Center,” he said in a statement Friday.

The search was focused Friday on three areas in the Indian Ocean: two larger areas where ships and aircraft were searching for any debris from the missing plane, and a third, much smaller area, where ships and sonar buoys were listening for pings from the black box.

The smaller search area, where the plane is thought to have settled on the ocean floor, lies about 1,040 miles northwest of the western Australian city of Perth.

The larger search areas, together amounting to more than 18,000 square miles, lie several hundred miles to the west of that point, where searchers say debris from the plane might have drifted.

Up to 12 military aircraft, three civilian aircraft and 13 ships are taking part in the search, but Houston said no debris was sighted and no objects were recovered from the ocean Friday.

“Today Ocean Shield is continuing more focused sweeps with the Towed Pinger Locator to try and locate further signals that may be related to the aircraft’s black boxes,” he said in a statement. “It is vital to glean as much information as possible while the batteries on the underwater locator beacons may still be active.”

He said three P-3 Orion surveillance planes will continue to work with Ocean Shield, dropping sonar buoys into the water to listen for sounds. Those buoys deploy hydrophones deep in the ocean and transmit audio signals back to the planes by radio.

Houston said a decision to deploy an underwater drone — the U.S. Navy’s Bluefin-21 — to explore the ocean depths could still be “some days away.”

The Bluefin-21 would be operating about three miles deep in the ocean, near its technical limit, and moves only about 4 mph. So it could take weeks to survey the ocean floor unless the search zone is narrowed further.

The disappearance of Flight 370, which was carrying 227 passengers and 12 crew members from Kuala Lumpur, Malaysia, to Beijing, has turned into one of the most confounding aviation mysteries on record.

The Boeing 777 is presumed to have crashed in the southern Indian Ocean, having traveled in its final hours in the opposite direction from its scheduled flight path. Malaysian officials say they believe the plane was deliberately steered off course by somebody on board, but they have not completely discounted the possibility of a mechanical failure.

Search officials have said the audio signals’ frequencies are consistent with tracking pings from a black box. But the officials have not conclusively determined that the signals came from the missing plane.

Two-thirds of the passengers on board the plane were Chinese, and their relatives have complained bitterly about how the investigation has been handled and what they see as a lack of transparency.

Abbott also met with Chinese President Xi Jinping in Beijing on Friday and briefed him on the search.

“This will be a very long, slow and painstaking process,” Abbott told Xi, the Associated Press reported.