PERTH (UPDATED) - Australian Prime Minister Tony Abbott said Friday he is "very confident" that signals detected in the search for Flight MH370 are from the aircraft's black box, whose batteries are waning fast more than a month after the plane vanished.
Abbott, speaking on a visit to China where two-thirds of the 239 on board the Malaysia Airlines jet came from, said he would brief Chinese President Xi Jinping more fully on the investigation later in the day.
Four signals linked to aircraft beacons have been detected by the Australian vessel Ocean Shield using US equipment, with the first two analysed as being consistent with those from aircraft flight recorders.
"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.
"It's been very much narrowed down because we've now had a series of detections, some for quite a long period of time.
"Nevertheless, we're getting to the stage where the signal from what we are very confident is the black box is starting to fade."
The ping-emitting beacons on the flight's data and cockpit voice recorders -- which have a normal battery lifespan of around 30 days -- are expected to die out as the days go by.
- 'Families need closure' -
The Australian-led search for the Boeing 777, which disappeared on March 8 en route from Kuala Lumpur to Beijing, is racing to gather as many signals as possible to determine an exact resting place before a submersible is sent down to find wreckage.
"We are confident that we know the position of the black box flight recorder to within some kilometres," Abbott said.
"But confidence in the approximate position of the black box is not the same as recovering wreckage from almost 4.5 kilometres beneath the sea, or finally determining all that happened on that flight."
He also offered support for relatives of the 153 Chinese passengers, saying he was grieving alongside them.
"I offer them the assurance that Australia will not rest until we have done everything we can to provide comfort and closure," the prime minister said.
Abbott's comments came as the Perth-based Joint Agency Coordination Centre (JACC) said 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 signal 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 upbeat announcement, Houston remained cautious.
"On the information I have available to me, there has been no major breakthrough in the search for MH370," he said.
The 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 plane has yet been found, the JACC said, despite the massive multinational air and sea operation.
Houston has stressed the need to find the wreckage to be absolutely certain of the plane's fate, and has repeatedly warned against unduly inflating hopes for the sake of the victims' relatives, whose month-long nightmare has been punctuated by false leads.
The JACC said that up to 12 military aircraft, three civil aircraft and 13 ships would join Friday's hunt.
The agency said it should not be long before a US-made autonomous underwater vehicle called a Bluefin-21 will be sent down to investigate, but has warned that it will have to operate at the very limits of its capability given the vast depths involved.
© 1994-2014 Agence France-Presse