The search team looking for the wreckage of the missing Malaysia Airlines plane have made an unexpected discovery.

Crews scouring the desolate waters off Western Australia for any sign of Flight MH370 found a cluster of objects that turned out to be a shipwreck.

Sonar equipment on board a search vessel in the Indian Ocean detected the items around 2.5 miles (4km) below the surface, the Australian Transport Safety Bureau said.

Although officials suspected the objects were not from the Boeing 777, which vanished last March with 239 people on board, a second ship sent down an unmanned sub to take a closer look to be sure.

It found a large number of small objects and several larger items, the biggest of which was 20ft (6 metres) long.

An analysis of photos taken by a camera that was also sent down has revealed the debris came from a previously unchartered shipwreck.

Marine archaeologists are examining the photos to see if they can identify the ship.

The pictures show an anchor and what appear to be lumps of coal.

Peter Foley, the ATSB's Director of the Operational Search For Flight 370, said: "It's a fascinating find, but it's not what we're looking for.

"We're not pausing in the search for MH370, in fact the vessels have already moved on to continue the mission."

Michael McCarthy, a senior maritime archaeologist at the West Australian Maritime Museum, said the wreckage was from a cargo ship built in the mid-to-late 19th century.

He said: "We've got quite a lot of stories about ships that sank in the Indian Ocean mid-voyage and you would be struggling to tell which is which unless you had a complete catalogue of all the ones lost."

Mr McCarthy said experts had predicted search crews would probably find the wreckage of some ships, given they at one time sank regularly due to old age or bad weather.

He added that it was difficult to identify the wreck without getting a closer look and knowing which ports it was travelling between.

Mr McCarthy said: "Being a fairly common type of cargo ship from the 19th century with no obvious cargo remains there, I doubt that anyone would pay the enormous cost of going down to look at it."

Officials announced last month they would expand the search area for Flight MH370 by another 23,000 square miles (60,000 square km) if the plane is not found by the end of May.

Crews have now covered 75% of the original search area and have moved into the southern portion of the expanded search zone to take advantage of the last of the good weather before winter takes hold.