Abbott says officials 'very confident' signals coming from black box

Australian Prime Minister Tony Abbott says he is confident signals being monitored in the search for missing Malaysia Airlines flight MH370 are coming from the plane's black box.  [Special coverage]

During his visit to China, Mr Abbott told reporters the search area for the aircraft had been very much narrowed.

"We have very much narrowed down the search area and we are very confident the signals are from the black box," he said.

But the Prime Minister said the signals were starting to fade.

"We are now getting to a stage where the signal from what we are very confident is the black box is starting to fade and we are hoping to get as much information as we can before the signal finally expires," he said.

Mr Abbott said he would wait for further briefings and his meeting with Chinese president Xi Jinping later today before saying more.

Soon after Mr Abbott spoke in Shanghai, the search's Joint Agency Coordination Centre (JACC) released a statement in which retired Air Chief Marshal Angus Houston said there had been "no major breakthrough".

"On the information I have available to me, there has been no major breakthrough in the search for MH370," he said.

"I will provide a further update if, and when, further information becomes available."

The JACC also said an early assessment of a fifth possible signal detected by a RAAF P3 Orion last night was not related to an underwater aircraft beacon.

"The Australian Joint Acoustic Analysis Centre has analysed the acoustic data and confirmed that the signal reported in the vicinity of the Australian Defence vessel Ocean Shield is unlikely to be related to the aircraft black boxes," Air Chief Marshal Houston said.

"Further analysis continues to be undertaken by Australian Joint Acoustic Analysis Centre."

Air Chief Marshal Houston said Ocean Shield was continuing more focused sweeps with its towed pinger locator to try and locate further signals that may be related to the aircraft's black boxes.

"It is vital to glean as much information as possible while the batteries on the underwater locator beacons may still be active," he said.

"A decision as to when to deploy the autonomous underwater vehicle will be made on advice from experts on board the Ocean Shield and could be some days away."

In recent days, four other signals had been detected in an area less than 40 kilometres apart by a US Navy towed pinger locator deployed by Ocean Shield.

This had seen the search area in the Indian Ocean already narrowed from 75,000 square kilometres to about 58,000 square kilometres.

The black boxes record cockpit data and may provide answers about what happened to the plane, which was carrying 227 passengers and 12 crew when it vanished on March 8.

It was flying thousands of kilometres off its Kuala Lumpur to Beijing route.

The batteries in the black boxes have reached the end of their 30-day expected life, making efforts to locate them on the murky ocean floor all the more critical.

Up to 12 military aircraft, three civil aircraft and 13 ships are involved in the search effort which has found no plane debris so far.

Efforts are now focused on two areas - aircraft and ships in the area about 2,240 kilometres north-west of Perth and a smaller area about 600 kilometres closer to the WA city.

The Ocean Shield picked up the acoustic signals around the smaller zone and dozens of acoustic sonobuoys were dropped there on Wednesday.

Once the final search area has been identified an autonomous underwater vehicle, the Bluefin-21 could be deployed to look for wreckage on the sea floor.

But Air Chief Marshal Houston has warned the deep water will mean any recovery effort of the missing plane will be a challenge.