However, shortly after Najib spoke, Paris Deputy Prosecutor Serge Mackowiak told reporters that experts examining the debris have reached only a "very strong presumption" that it is from the missing Malaysian jetliner, adding that their finding still needed to be confirmed.
The Australian government, which leads the seabed search for wreckage west of Australia, is also less certain than Malaysia, saying in a statement that "based on high probability, it is MH370."
Australian officials are withholding criticism of Najib's announcement, with Australian Transport Minister Warren Truss saying Malaysia, as the government in charge of the investigation, had the right to make that call.
"We respect the view that they believe they have sufficient evidence to make a categorical statement of that nature," Truss told reporters. "The French inquiry, of course, has not been quite so conclusive."
Privately, however, there were questions about why Najib had moved forward with the statement before all officials had agreed. An Australian government official, speaking on condition of anonymity because he wasn't authorized to speak publicly, said Malaysia wasn't supposed to make the announcement, and had gone out on its own making a conclusive statement before getting the evidence to back it up.
The plane piece was discovered July 29 on Réunion, a volcanic island of 850,000 people that is a part of France, in the Indian Ocean east of Madagascar.
Réunion is roughly 2,300 miles west of the broad expanse of the southern Indian Ocean off Australia where search efforts have focused, but officials and experts said currents could have carried wreckage that distance.
The Boeing 777 jetliner disappeared on March 8, 2014, while flying from Kuala Lumpur, Malaysia, to Beijing, with 239 people on board. It is believed to have crashed in the Indian Ocean, but the reason remains one of aviation's biggest mysteries.
Malaysia Airlines released a statement on Wednesday, conveying its "deepest sorry" to the victims' families.
"This is indeed a major breakthrough for us in resolving the disappearance of MH370. We expect and hope that there would be more objects to be found which would be able to help resolve this mystery," the statement read.
Al Jazeera and Reuters