Australian leader Tony Abbott says authorities are confident that signals heard in the Indian Ocean are coming from the "black box" flight recorders of the missing Malaysia Airlines plane.Speaking in China, he said teams had "very much narrowed" the search area.An Australian vessel has on four occasions picked up signals consistent with flight recorders, officials say.Meanwhile, China has postponed delivery of two pandas to Malaysia out of respect for relatives of the missing.The giant pandas had been due to be transferred next week to mark the 40th anniversary of diplomatic relations between the two countries.Malaysia's environment minister G Palanivel said: "During this difficult time, it seems inappropriate to arrange for the sending off and the arrival of pandas in Malaysia".Narrowed searchAn Australian vessel, the Ocean Shield, has been using a US Navy towed pinger locator to listen out for signals, which were detected twice over the weekend and twice on Tuesday. Speaking in China during an official visit, Mr Abbott said search teams needed as much information as possible from the acoustic signals before the black-box batteries ran out."It [the search area] has been very much narrowed down because we've now had a series of detections, some for quite a long period of time,'' Mr Abbott said."Nevertheless, we're getting to the stage where the signal from the black box is starting to fade."He also said that officials were confident that they knew "the position of the black box flight recorder to within some kilometres"."Still, confidence in the approximate position of the black box is not the same as recovering wreckage from almost 4.5km (2.6 miles) beneath the sea or finally determining all that happened on that flight."On Thursday an Australian aircraft picked up an audio signal in the same area as the four previous detections but officials now believe it is unlikely to be related to the black boxes.Air Chief Marshal Angus Houston, who heads the agency overseeing the search, said there had been "no major breakthrough in the search for MH370".And he has cautioned that search work using the towed pinger locator will continue until officials are sure that the black-box batteries - which last about a month - have run out.At that point the Bluefin 21 submersible drone will be sent down to search for wreckage on the sea floor, but this could be a laborious and pain-staking task made more difficult by the presence of silt.On Friday, up to 15 aircraft and 13 ships were involved in the search, which was targeting a reduced area of 46,713 sq km (18,036 miles).As the Ocean Shield continues to listen for acoustic signals, ships and aircraft are combing another area for possible debris from the plane, based on analysis of ocean drift, but nothing has yet been sighted.Possible causesMalaysia Airlines Flight 370 was flying from Kuala Lumpur to Beijing on 8 March when it lost contact with air traffic controllers and vanished from radar.Of the 239 people on board, 153 were Chinese. Many relatives have been angered by what they perceive to be the Malaysian authorities' early misguided response to the flight's disappearance.The plane disappeared over the South China Sea, between Malaysia and Vietnam, but it was a week before the search was widened based on evidence taken from radar and satellite tracking.Officials now believe it crashed into the southern Indian Ocean, west of the Australian city of Perth, but are still no clearer as to why the plane strayed so far off course.The backgrounds of both passengers and crew have been scrutinised as officials consider hijacking, sabotage, pilot action or mechanical failure as possible causes. ANN.Az