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The Australian Transport Safety Bureau (ATSB) has assessed the material that washed up on the coast 10km east of the town of Augusta, near the southwestern tip of Australia, the bureau's spokesman said.
"It's considered highly unlikely to be from MH370," spokesman Tony Simes said.
ATSB commissioner Martin Dolan earlier told Australian Broadcasting Corporation radio that the bureau had examined detailed photographs of the debris and was satisfied it was not a lead in the hunt for the plane.
"We're not seeing anything in this that would lead us to believe that it is from a Boeing aircraft," he told ABC Radio.
Authorities have given no details on the material, which was the first discovery of suspected debris in weeks and the first since the detection of what were believed to be signals from the plane's black box flight recorder on April 4.
Seabed scans of a 10 km zone off the west coast of Australia have failed to turn up any wreckage, but Malaysia and Australia have vowed to plough on with the search for the plane that went missing on March 8 with 239 people on board.
The U.S. navy drone that has been scouring the seabed is due to end its first full mission in the southern Indian Ocean within days.
Malaysian acting Transport Minister Hishammuddin Hussein said the search authorities would need to "regroup and restrategise" if nothing was found in the current search zone, but said the search would "always continue".
"I can confirm in fact we are increasing the assets that are available for deep-sea search," he told a news conference in Kuala Lumpur on Wednesday, adding that the government was seeking help from state oil company Petronas, which has expertise in deep-sea exploration.