The search zone for flight MH370 has narrowed down to 1300 square kilometres after more audio signals consistent with the plane’s black box were detected from the depths of the Indian Ocean.

The furthest distance between the four most recent pings is about 25 km, a dramatic narrowing of the focus zone, considering the Indian Ocean search for the missing Malaysia Airlines plane began with a 305,000 square kilometre area to scour.

“Considering what we started from, which was a huge chunk of the Indian Ocean, it’s certainly a remarkable reduction,” one of the search coordinators, US Navy Captain Mark Matthews, said on Wednesday.

“If you bounded all those detections in a circle, you’re still talking 1300 square kilometres.”

Search co-ordinator and former defence force chief Angus Houston said a signal was picked up by Australian vessel Ocean Shield on Tuesday afternoon and was held for five minutes and 32 seconds.

MH370 ping locations. Picture: AMSA

A second signal, detected by the same ship almost six hours later, was held for about seven minutes.

Retired air chief marshal Houston described them as “great” and “encouraging” leads.

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

“For the sake of the 239 families, this is absolutely imperative.”

For the sake of the 239 families, this is absolutely imperative.

“I’m not going to confirm anything until someone lays eyes on the wreckage.”

Mr Houston said he was optimistic wreckage would be found “in the not too distant future”.

The signals came three days after Ocean Shield picked up a signal that held for some two hours and 20 minutes, and another that held for about 13 minutes.

Ocean Shield’s search zone, more than 2000 kilometres northwest of Perth, was now being combed in tighter patterns, he said.

A map of the search area for MH370. Picture: AMSA

Mr Houston said the pinger detector towed by Ocean Shield would continue to be used until it was absolutely clear the black box beacon’s battery was dead.

It is 33 days since the plane went missing en route from Kuala Lumpur to Beijing – three days beyond the battery’s life – but they are known to last several days longer.

“We are looking for transmissions that are probably weaker than they would be earlier on,” Mr Houston said.

Once the battery was declared expired, the automated underwater vessel Bluefin-21 would be deployed and begin relaying sidescan sonar data and images from the silty sea floor, some 4.5km from the surface.

“I don’t think that time is very far away,” Mr Houston said.

The pinger locator can scan an area six times faster than the Bluefin-21.

Mr Houston added there were no plans to send more ships and pinger locators to the search area because it was crucial to keep the waters as quiet as possible.

Two thrusters are operating at the back of the Ocean Shield to propel the vessel, but everything else has been turned off to optimise the device’s operation.

He also said the separate effort in the southern part of the search zone involving Chinese ship Hai Xun 01 and British ship HMS Echo had not picked up any further audio signals since a detection was reported on Sunday.

The search continues for debris on the surface of the ocean.

-AAP