Media playback is unsupported on your device Media caption Air Chief Marshal Angus Houston: "Until we stop the pinger search, we will not deploy the submersible"

An Australian vessel searching for the missing Malaysia Airlines plane has detected signals consistent with those from "black box" flight recorders.

The Ocean Shield ship picked up the signal twice, once for more than two hours, said Angus Houston, a retired air chief marshal leading the search.

He called it the "most promising lead" so far.

But he said more information was needed: "We haven't found the aircraft yet and we need further confirmation."

Malaysia's acting transport minister, Hishammuddin Hussein, said he had been briefed by Air Chief Marshal Houston and was "cautiously hopeful that there will be a positive development in the next few days if not hours".

The plane, carrying 239 people, was flying from Kuala Lumpur to Beijing on 8 March when it disappeared. Malaysian officials say they believe it crashed in the southern Indian Ocean.

'Two distinct returns'

ACM Houston said the signals were detected using the towed pinger locator deployed on the Ocean Shield.

Image copyright Reuters Image caption The signals were heard by the towed pinger locator deployed by the Ocean Shield

Two separate detections occurred, he said. The first was held for two hours and 20 minutes before being lost.

Analysis "Lottery odds" is how one oceanographer described to me how incredible it would be if they had actually stumbled over distress signals from the black boxes. This listening equipment trickles along at just a few miles an hour and they were searching an area the size of Britain. Now they are racing against the clock to try to pick up the signals again so they can pinpoint a better location, and send a drone down to look for parts of the aircraft. The battery on this pinger will be starting to go flat, which cuts the range. One very interesting part of these findings is the fact that they seemed to hear two separate pings on one of their sweeps. There are two pingers on board the plane, one for each black box. Could the noise be something else? Well, the frequency of black box pings, 37.5 kHz, is designed to be unique in the water. It's an unusual frequency, so that it's easier to find. But the ocean's an incredibly noisy place and people and equipment can make mistakes.

The ship then turned around and on the return leg detected the signal again for 13 minutes.

"On this occasion two distinct pinger returns were audible. Significantly this would be consistent with transmissions from both the flight data recorder and the cockpit voice recorder," ACM Houston said.

"I'm much more optimistic than I was a week ago," he said.

"We are now in a very well defined search area, which hopefully will eventually yield the information that we need to say that [Malaysia Airlines flight] MH370 might have entered the water just here."

The Ocean Shield was still in the area, about 1,040 miles (1,680 km) north-west of the Australian city of Perth, but had not been able to reacquire the signals since, he said.

The position of the signals needed to be fixed, ACM Houston said. Once that happened, the Ocean Shield could lower the Bluefin 21 underwater autonomous vehicle to try to locate wreckage on the sea floor.

The signal had been heard in sea with a depth of 4,500m, he added, which was at the limit of the capability of the Bluefin 21.

He cautioned that the next steps would take time.

"It could take some days before the information is available to establish whether these detections can be confirmed as being from MH370," he said. "In very deep oceanic water, nothing happens fast."

The search operation is in a race against time as the flight recorders' batteries are due to run out, meaning a signal would no longer be emitted.

A Chinese search vessel, Haixun 01, also said it briefly heard signals over the weekend in a different search area.

Media playback is unsupported on your device Media caption Navy divers searched the area near the signal - courtesy Australian Defence Force

Media playback is unsupported on your device Media caption Families hold a candle-lit vigil in Kuala Lumpur, as the "black box" hunt continues

Those signals are now being investigated with the help of a British naval vessel, HMS Echo, which is equipped with sophisticated sound-locating equipment.

Reports said the crew of the Chinese ship had been using a sonar device called a hydrophone to pick up sounds.

Experts said it was technically possible but unlikely that the sounds heard with this equipment related to the missing plane.

Chris Portale, a director of the US company Dukane which makes the device that emits signals from flight recorders, said looking for the Malaysian plane's "black boxes" was like "looking for a suitcase on the side of a mountain" but under water.

But he said he thought searchers were now in the right area and had a "very good hope" of spotting debris, if the signal was from the aircraft.

"I believe they have got three to four more days of good, solid output [from the flight recorders]," he told the BBC's Today Programme.

Media playback is unsupported on your device Media caption Richard Westcott reports on the use of a pinger locator to find a black box

So far, not a single piece of wreckage has been found from the missing plane, but officials have concluded - based on satellite data - that it ended its flight in the sea to the west of Perth.

Officials do not know why it lost contact with air traffic controllers and ended up so far from its intended path.

The backgrounds of both passengers and crew have been investigated but to date officials are not thought to have discovered any concrete evidence about what might have caused the plane to disappear.