An Australian vessel searching for the missing Malaysia Airlines plane has detected signals consistent with those from aircraft black boxes.

Australian defence vessel Ocean Shield acquired the signal twice, once for more than two hours, Australia said.

Air Chief Marshal Angus Houston, who is leading the search, 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."

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 that the signals were detected using the towed pinger locator deployed on the Ocean Shield.

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

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.

The Ocean Shield was still in the area 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. These signals are also being investigated.

Not a single piece of wreckage has yet 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 the Australian city 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.