By Swati Pandey

SYDNEY (Reuters) - Australian authorities searching for the wreckage of a Malaysia Airlines jet should urgently re-investigate two areas in the remote Indian Ocean where sonar pictures show what could be debris from the plane, deep-sea search experts said on Tuesday.

An Australian-led underwater search, the most expensive in aviation history, has so far found no trace of Malaysia Airlines Flight MH370, which went missing with 239 passengers and crew during a flight from Kuala Lumpur to Beijing in March 2014.

However, U.S. firm Williamson & Associates said images of the southern Indian Ocean floor released by the Australian Transport and Safety Bureau (ATSB) this month bore a striking similarity to the underwater debris field Air France Flight 447 left on the Atlantic Ocean floor when it crashed in 2009, killing all 228 passengers and crew.

The ATSB provided the images of the two sites as part of an update in which it also said a re-scan of the areas indicated they were not related to MH370.

Experts involved in past deep-water searches have said the hunt could easily miss the plane because Dutch company Fugro NV was using inappropriate technology and inexperienced personnel for the highly specialized task.

Williamson and Associates "believes the target (sonar image) bears the hallmarks of a classic high-impact debris trail similar to other wrecks it has located", it said in a research paper obtained by Reuters.

"We believe that any target with these characteristics should be investigated as a high priority," it said.

The ATSB said in a written response to Reuters that geophysicists, sonar data specialists and its quality assurance team were satisfied that the structures in the sonar records were "consistent with the surrounding geological formations".

"Based on analysis of all of the data, there are no indications that there is anything possessing the characteristics of an aircraft debris field," it said.

Fugro did not respond to requests for comment.

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The ATSB said in a Sept. 23 update the ship Fugro Discovery would resurvey "several" sites identified by sonar contact as being of possible interest in the 120,000 sq km search area.

Other experts say the ATSB could have instead used an autonomous underwater vehicle (AUV) to resurvey the sites. An AUV provides the most accurate search readings but cannot be deployed in rough winter weather.

A piece of the plane found washed up on the French island of Reunion in July provided the first direct evidence that the plane had crashed into the sea. No further trace has been found.

Williamson was one of the rejected bidders for the contract to search for MH370.

(Additional reporting by Jane Wardell; Editing by Paul Tait)