Malaysian Prime Minister Najib Razak says analysis has determined that plane debris found on the Indian Ocean island of Réunion belongs to missing Malaysia Airlines Flight MH370.

"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 … is indeed MH370," Razak told reporters early Thursday local time in Kuala Lumpur.

A French prosecutor was more cautious, saying there is a "very strong presumption" that the plane debris belonged to the missing Malaysia Airlines flight, which disappeared with 239 people on board 515 days ago. 

Serge Mackowiak told a Paris news conference that investigators came to their conclusion for two reasons: Boeing's confirmation that the wing piece discovered is from a Boeing 777, the model of the missing plane; and Malaysia Airlines' information about the specific plane.

"This very strong presumption will have to be confirmed by other analysts who will begin their work tomorrow," he said of the debris found on the French island in the Indian Ocean.

In a statement, Malaysia Airlines said the debris was confirmed to be from Flight MH370 by the French agency that investigates air crashes, known as the BEA, the Malaysian investigation team, a technical representative from China and the Australian Transportation Safety Bureau in Toulouse, France.

Missing Malaysia Plane

This flaperon from the trailing edge of a Boeing 777 wing is believed to be from Malaysia Airlines Flight MH370, which disappeared March 8, 2014, with 239 people on board. (Lucas Marie/Associated Press)

"Family members of passengers and crew have already been informed and we extend our deepest sympathies to those affected," it said.

The statement said 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."

But Sara Weeks, the sister of New Zealander Paul Weeks who was on Flight MH370, criticized the mixed messages on whether the flaperon had indeed been confirmed as part of the missing plane.

"It's somewhat frustrating," she said from Christchurch, New Zealand, where she lives.

"Why the hell do you have one confirm and one not? Why not wait and get everybody on the same page so the families don't need to go through this turmoil?" Weeks said.

Jacquita Gomes, who lost her husband Patrick Gomes, a flight attendant on board the aircraft, said even if the wreckage is from Flight MH370, the families are far from a sense of closure.

"Although they found something, you know, it's not the end. They still need to find the whole plane and our spouses as well. We still want them back," she said.

'A needle in a field full of haystacks'

Simon Boxall, a tidal drift expert with Southampton University in England, told CBC News that the discovery of the wing piece shows that the original search area was correct. The debris turned up where you would expect it to at the right time.

APTOPIX Malaysia Missing Plane

Malaysian Prime Minister Najib Razak announced that experts have confirmed debris found last week was part of Malaysian Airlines Flight MH370. (Vincent Thian/Associated Press)

However, it won't help narrow the search area where the plane's black boxes likely sank.

"They're definitely looking in the right place, but the problem is the right place is huge," he said. "It is looking for a needle in a field full of haystacks."

Even so, it's possible this finding could lead to more discoveries, according to ocean expert Tim Taylor, president of Tiburon Subsea Services,

"Now that they have a physical piece of the plane they can start searching for these clues of how it went down and how it broke up and that may lead them to adjust their search area based on some knowledge that it crashed in the ocean, or didn't crash in the ocean," he told CBC News.

Missing Malaysian Plane

Family members of the missing on Malaysia Airlines Flight 370, Jacquita Gomes, wife of Patrick Gomes, in-flight supervisor, and Melanie Antonio, wife of Andrew Nari, chief steward, check a mobile phone in Gomes's house outside Kuala Lumpur on Wednesday. (Joshua Paul/Associated Press)

John Page, an aircraft design expert at the University of New South Wales in Australia, thinks the latter possibility is the most likely, telling The Associated Press his conclusion is that the missing Boeing 777 broke up, most likely when it hit the water.

He said that while the main body of the plane is likely to have sunk, he thinks other small, lightweight parts attached to the wings and tail may could still be afloat — pieces like the flaps, elevators, ailerons and rudders.

"I'm certain other bits floated," he said. "But whether they've washed up anywhere is another question. The chances of hitting an island are pretty low."

With files from Reuters and The Associated Press