Searchers engaged in a race against time to pinpoint "pings" from the missing Malaysian airliner's black boxes on Thursday detected a possible fifth signal, fuelling hopes that wreckage will soon be found.

The beacons on flight MH370's data and cockpit voice recorders are due to fade, more than a month after the Boeing 777 vanished. So the Australian-led search is vying to determine an exact location before sending down a submersible to plumb the Indian Ocean depths.

The Perth-based Joint Agency Coordination Centre (JACC) said the latest ping was detected Thursday afternoon by an Australian air force P-3C Orion surveillance plane, which has been dropping dozens of sonar buoys into the remote waters of the search zone.

"The acoustic data will require further analysis overnight but shows potential of being from a made-made source," JACC chief Angus Houston said in a statement.

The Australian ship Ocean Shield, bearing a special US Navy "towed pinger locator", is now focused on a far smaller area of the Indian Ocean 2,280 kilometres (1,400 miles) northwest of Perth where it picked up two fresh signals Tuesday.

Those transmissions matched a pair of signals logged over the weekend.

"When you put those two (sets of pings) together, it makes us very optimistic," US Seventh fleet spokesman commander William Marks said earlier on CNN, adding that the search was getting "closer and closer".

"This is not something you find with commercial shipping, not something just found in nature -- this is definitely something that is man-made, consistent with what you would find with these black boxes."

Marks said he expected the pings to last "maybe another day or two" as the batteries powering the black box beacons fade after their normal lifespan of about 30 days.

No floating debris from the Malaysia Airlines aircraft, which disappeared on March 8 with 239 people aboard, has yet been found despite days of exhaustive searching by ships and aircraft from several nations.

- Investigation still 'inconclusive' -

Houston said the high-tech underwater surveillance was meant to define a reduced and more manageable search area in depths of around four kilometres (2.5 miles), but he acknowledged that time was running out.

"I believe we are searching in the right area but we need to visually identify the aircraft before we can confirm with certainty that this is the final resting place of MH370," he said Wednesday.

Houston again urged 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.

But he voiced renewed optimism.

"They (experts) believe the signals to be consistent with the specification and description of a flight data recorder," he said.

No other ships will be allowed near the Ocean Shield as it must work in an environment as free of noise as possible, but up to 10 military aircraft, four civil planes and 13 ships were taking part in surface searches in the region on Thursday, the JACC said.

Houston said it would not be long before a US-made autonomous underwater vehicle called a Bluefin-21 would 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.

In Malaysia, Home Minister Zahid Hamidi said there was "no conclusive evidence yet" from the continuing investigation into what caused the plane to divert from its Kuala Lumpur-Beijing route.

Zahid, who oversees law enforcement, said around 180 people had been interviewed, including relatives of passengers and crew as well as airline ground staff and engineers.

"We are filtering all the information. When the evidence is conclusive then we will let the media know about it," he said.

A number of 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 in its suspected Indian Ocean crash site.

But no evidence has emerged to bolster any theory.

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