Sub set to relaunch underwater quest for MH370

A mini-sub hunting for missing Malaysia Airlines Flight MH370 will be back in the search zone within days, an official said Saturday, as the Australian ship carrying the device prepared to leave on the mission.

Australian vessel Ocean Shield is carrying the US Navy Bluefin-21 mini-sub which had been scouring the seabed for the plane which disappeared on March 8 carrying 239 people until it docked to resupply early this week.

In this photo taken on May 5, 2014 Australian navy vessel Ocean Shield is pictured berthed at Fleet Base West near Perth as it prepared to resupply and undertake routine maintenance. (AFP Photo)

Ocean Shield was due to head back Saturday to the remote area of the Indian Ocean where transmissions believed to have come from the plane's black box recorders were heard last month, a journey expected to take three days.

Once in the area, Ocean Shield will be able to deploy the Bluefin-21 to look for "any non-normal items, any metallic items", US Navy Captain Mark Matthews told the Australian Broadcasting Corporation.

"They'll either find something or they won't, that's about all I can box in, but what you do is you go look at your best indications and you pursue them until they're exhausted," he said.

Matthews said it was impossible to know for sure whether the signals picked up were from the plane's black box.

"It is certainly a man-made signal, but what it's from, I can't look at it and positively say, 'Hey that's an underwater locator beacon'," he said.

Extensive air and sea searches over vast stretches of the Indian Ocean have failed to find any sign of MH370 which mysteriously diverted from its Kuala Lumpur to Beijing route and is thought to have crashed far off Australia's west coast.

Australia, which is leading the search, has stressed that it believes it is looking in the right area based on satellite communications from the plane.

Officials have scaled back the air and sea searches, and have said that an intensified undersea mission will begin once new, more sophisticated equipment can be obtained to search at depths of more than 4,500 metres (15,000 feet).

The ocean bed in the prospective search zone is several kilometres deep and largely unmapped, meaning specialist sonar equipment and other autonomous vehicles are needed.

Until these can be deployed, the Bluefin-21 will continue the search while oceanographic work will also be done.

Meanwhile international experts are re-examining satellite imagery and all the data collated so far to try to pinpoint a more precise location for the search.