Australian Prime Minister Tony Abbott announced Friday he was "very confident" that a series of signals detected in the remote Indian Ocean were from the black box of missing Malaysian Flight MH370.
The comments came as the Australian-led operation raced to gather as many signals as possible to determine an exact resting place for the Malaysia Airlines Boeing 777 before sending down a submersible to plumb the depths.
"We have very much narrowed down the search area and we are very confident the signals are from the black box," Abbott said in Shanghai.
He said he did not want to reveal any more before he briefed the Chinese leadership later in the day. Two-thirds of the 239 people onboard Flight MH370 were Chinese.
But he elaborated on his confidence.
"It's been very much narrowed down because we've now had a series of detections, some for quite a long period of time," he said of the search.
"Nevertheless, we're getting to the stage where the signal from what we are very confident is the black box is starting to fade.
"We are hoping to get as much information as we can before the signal finally expires."
The ping-emitting beacons on the flight's data and cockpit voice recorders are expected to fade as days go by, more than a month after the plane vanished en route from Kuala Lumpur to Beijing.
The Perth-based Joint Agency Coordination Centre (JACC) said Friday that initial analysis had determined that a fifth signal picked up on Thursday afternoon by a P-3C Orion surveillance plane did not appear linked to aircraft beacons.
The acoustic data was "unlikely to be related to the aircraft black boxes" JACC chief Angus Houston said. "Further analysis continues to be undertaken."
The Orion was flying close to the area where two signals were detected at the weekend and two more on Tuesday by Australian ship Ocean Shield.
The vessel is dragging a US Navy "towed pinger locator" to listen for emissions from black boxes.
"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," Houston said.
"It is vital to glean as much information as possible while the batteries on the underwater locator beacons may still be active."
He added that a decision to deploy a submersible sonar device "could be some days away".
- Still no debris spotted -
Despite Abbott's statement, Houston noted: "On the information I have available to me, there has been no major breakthrough in the search for MH370."
JACC said the search area for Friday had been further reduced to two zones totalling 46,713 square kilometres (18,06 square miles).
The core of the search is now 2,312 kilometres (1,436 miles) northwest of Perth.
Friday's weather forecast in the search zone was for 10-15 knot southerly winds with isolated showers, seas swells of one to 1.5 metres (three to five feet) and visibility of five kilometres during the showers.
No floating debris from the Malaysia Airlines aircraft has yet been found, JACC said, despite the massive multinational air and sea operation.
JACC says the high-tech underwater surveillance is intended to define a reduced and more manageable search area in depths of around four kilometres (2.5 miles).
Houston has stressed the need to find the wreckage and urged repeatedly against unduly inflating hopes, for the sake of the families of missing passengers and crew who have endured a month-long nightmare punctuated by a number of false leads.
No other ships are being allowed to sail near the Ocean Shield as it must work in an environment as free of noise as possible.
But JACC announced that up to 12 military aircraft, three civil aircraft and 13 ships would join Friday's hunt.
JACC says says it should not be long before a US-made autonomous underwater vehicle called a Bluefin-21 will be sent down to investigate, but has cautioned that it will have to operate at the very limits of its capability given the vast depths involved.
Numerous theories have been put forward to explain MH370's baffling disappearance.
They include a hijacking or terrorist attack, a pilot gone rogue or a sudden catastrophic event that incapacitated the crew and left the plane to fly for hours until it ran out of fuel.