Debris found on an Indian Ocean island last week is from MH370, Malaysia's prime minister has said, confirming for the first time that the plane which mysteriously disappeared 17 months ago had crashed.

Najib Razak's announcement in Kuala Lumpur on Wednesday ended an agonising wait for families of the 239 passengers and crew of the Malaysia Airlines flight who had demanded concrete proof of what happened to their missing loves ones.

The piece of debris, a wing component called a flaperon, was found last week on a beach on the French island La Reunion, near Madagascar.

It was flown to the French city of Toulouse where it was examined on Wednesday by French and Malaysian technical experts, and representatives from Boeing to determine any link to MH370.

Many relatives accuse Najib's government and Malaysia Airlines of a bungled response to the disaster, possible cover-up and insensitive treatment of families, charges that are vehemently denied.

"Today, 515 days since the plane disappeared, it is with a very heavy heart that I must tell you that an international team of experts has conclusively confirmed that the aircraft debris found on Reunion Island is indeed from MH370," Najib said.

"We now have physical evidence that, as I announced on 24th March last year, flight MH370 tragically ended in the southern Indian Ocean."

Shortly afterwards, a French prosecutor made a more cautious statement over the issue.

"There exists a very high probability that the flaperon indeed belongs to flight MH370," Serge Mackowiak, deputy Paris prosecutor, said in the French capital.

He was unable to say when more detailed results could be available.

Next-of-kin, investigators and the aviation industry are still left with the question of what caused the Boeing 777 aircraft to inexplicably divert on a flight from Kuala Lumpur to Beijing on March 8, 2014.

Al Jazeera's Sohail Rahman, reporting from Kuala Lumpur, said that the families of the passengers have accused the government and Malaysia Airlines of not sharing with them all the information they have.

The aircraft apparently veered out over the Indian Ocean, flying for hours after its communications and tracking systems were shut off, in what remains one of the biggest mysteries in the history of flight.

Najib gave no indication that the analysis of the debris yielded any clues into the cause of the disappearance.

"I would like to assure all those affected by this tragedy that the government of Malaysia is committed to do everything within our means to find out the truth of what happened," he said.

"MH370's disappearance marked us as a nation. We mourn with you, as a nation."