Investigators looking into the disappearance of flight MH370 have cast doubt on a theory it crashed because of a fire on board.

The Malaysia Airlines plane vanished on March 8th, 2014, en-route from Kuala Lumpur to Beijing with the loss of 239 lives.

Wreckage hunter Blaine Gibson last week hand delivered five pieces of blackened debris found on Madagascar to the Australian Transport Safety Bureau.

The bureau said it was not yet sure whether the pieces were from the Boeing 777 that is thought to have plunged into the Indian Ocean southwest of Australia.

In a statement, it said: "At this stage it is not possible to determine whether the debris is from MH370 or indeed even a Boeing 777.

"Further work will be undertaken in an attempt to determine the origin of the items, specifically whether they originated from a Boeing 777 aircraft."

But, it added, a preliminary examination found that two fiberglass-honeycomb pieces were not burnt, but had been discoloured by a reaction to resin that had not been caused by exposure to fire or heat, the statement said.

It said there were three small areas of heat damage on one of the pieces which gave off a smell of burning. However, the smell suggested the heat damage was recent.

The statement explained: "It was considered that burning odours would generally dissipate after an extended period of environmental exposure, including salt water immersion, as expected for items originating from the missing plane."

The American wreckage collector has found 14 pieces of debris that could potentially come from the missing plane.

He had suggested the darkened surfaces of the latest debris could be evidence of a fire that caused the plane to crash.

A sonar search of 120,000 square kilometres (46,000 square miles) of seabed which is calculated to be the most likely crash site in the southern Indian Ocean is almost complete and no trace of the plane has been found.