Australian Prime Minister Tony Abbott said 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.

But a fifth signal picked up by a plane on Thursday is now thought unlikely to be linked to flight MH370.

The Malaysian plane vanished on 8 March, with 239 people on board. It was flying from Kuala Lumpur to Beijing when it lost contact with air traffic controllers.

Based on satellite data, officials believe it crashed into the southern Indian Ocean, west of the Australian city of Perth, far from its intended flight path.

An Australian vessel, the Ocean Shield, has been using a US Navy towed pinger locator to listen out. Signals 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's [the search area] 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 the Australian Joint Acoustic Analysis Centre had analysed the data and confirmed that the signal was "unlikely to be related to the aircraft black boxes", said Air Chief Marshal Angus Houston, who heads the agency overseeing the search.

He said that there had been "no major breakthrough in the search for MH370".

ACM Houston 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 square kilometres.

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.

"Yesterday there were no sightings reported by search aircraft or objects recovered by ships," the co-ordinating agency said in a statement on Friday morning.

Investigators still do not know why MH370 strayed so far off course, after disappearing over the South China Sea between Malaysia and Vietnam.

The backgrounds of both passengers and crew have been scrutinised as officials consider hijacking, sabotage, pilot action or mechanical failure as possible causes. (BBC)