The hunt for missing Malaysia Airlines Flight MH370 has uncovered a previously uncharted shipwreck deep underwater, leading officials to say Wednesday that if the plane is in their search zone they will find it.

The Australian-led team is scouring the southern Indian Ocean seabed in hope of finding the final resting place of MH370, which vanished on March 8, 2014 en route from Kuala Lumpur to Beijing.

No wreckage from the flight, which was carrying 239 people, has ever been found in one of aviation's greatest mysteries.

In a search update, the Australian Transport Safety Bureau said it spotted "multiple small bright reflections" on the otherwise featureless seabed which warranted closer inspection.

Data from a high-resolution sonar scan using an autonomous underwater vehicle revealed spots worth probing, mostly about the size of a cricket ball but some larger, at 3,900 metres (12,795 feet) down.

While the objects appeared to be of man-made origin, they failed to have all the characteristics of a typical aircraft debris field so authorities sent in an underwater camera which discovered the shipwreck.

"It's a fascinating find," said Peter Foley, director of the operational search for MH370. "But it's not what were looking for."

Images clearly showed an anchor, along with other objects the searchers said were man-made as well as what are thought to be lumps of coal.

"Obviously, we're disappointed that it wasn't the aircraft, but we were always realistic about the likelihood," added Foley in a statement.

"And this event has really demonstrated that the systems, people and the equipment involved in the search are working well. It's shown that if there's a debris field in the search area, we'll find it."

- Expanded search underway -

The search for the aircraft has been a complex undertaking, with Australia concentrating on a remote area of the southern Indian Ocean far off its west coast, originally focusing on a 60,000 square kilometres (23,166 square miles) zone.

But with more than 75 percent searched without success so far, the hunt has now been expanded into a 120,000 square kilometre area as announced by Malaysia, Australia and China in April. Most of those on board MH370 were Chinese.

The Joint Agency Coordination Centre (JACC), managing information on the search, said it was widening now to take advantage of the weather ahead of expected worsening conditions during the southern hemisphere winter.

The search zone has been determined by analysing data from satellite signals which indicate the plane went down in the Indian Ocean after mysteriously diverting.

"Expert advice is that the highest probability of locating the aircraft is within the 120,000-square-kilometre search area," JACC said.

"Beyond that, it is not possible to refine the search area to one of greater likelihood."

Several vessels are taking part in the hunt, although the onset of poor weather has seen the autonomous underwater vehicle withdrawn following the sonar scan.

"The deteriorating weather has brought sea conditions which are beyond the safe launch and recovery limitations of the autonomous underwater vehicle," the centre said, but added that it would remain available at short notice if needed.

JACC said the shipwreck was found after high-resolution data revealed a large number of sonar contacts lying very close to the seabed.

"The majority of the contacts were comparatively small -- around the size of a cricket ball -- interspersed with a few larger items, the biggest being box shaped and approximately six metres in its longest dimension," it said.

Poor weather conditions then prevented searchers from sending down an underwater camera for several days, before it ultimately revealed the wreck, with the imagery provided to marine archaeologists for possible identification.