The plane plunged into the Atlantic Ocean shortly after the pilot reported an emergency, killing all 229 people on board.

Vic Gerden, the chief crash investigator, said: "There are some signs of heating on some of the small pieces of the wreckage retrieved."

"The limited amount of wreckage retrieved to date indicates this area of heat stress is in the cockpit," he added.

However, he said it was much too early to draw any conclusions.

The pilot reported smoke in the cockpit some 16 minutes before the fatal crash, leading to speculation that an electrical failure may have caused the crash.

Correspondents say signs of heat damage may support the theory that there were electrical problems.

The plane had been intending to make an emergency landing at Halifax when the crash occurred.

Radar signals showed that the airliner began flying off course in a rapidly descending loop over the sea.

Hopes that the flight data recorder - or "black box" - would yield vital information have been dashed.

The recorder, which was recovered this week, has no information for the last six minutes of the flight.

Efforts are continuing to retrieve the cockpit voice recorder, which has been located on the sea bed.