Relatives of those missing on Malaysia Airlines flight MH370 have vented anger at apparent mixed signals over whether part of the plane has been found.

Malaysian PM Najib Razak said experts in France had "conclusively confirmed" the wing part found on an island in the Indian Ocean was from the aircraft.

But French investigators stopped short of confirming the link, only saying it was highly likely.

Chinese relatives staged a protest outside the airline's Beijing offices.

The Boeing 777 was travelling from Kuala Lumpur to Beijing on 8 March 2014 when it vanished from radar. It had 239 people on board, most of them Chinese.

Debris found on the remote French island of Reunion a week ago - a wing part known as a flaperon - was the first possible physical trace of the aircraft.

Experts in the French city of Toulouse resumed their tests on the object on Thursday.

"Why the hell do you have one confirm and one not?" asked Sara Weeks of Christchurch, New Zealand, whose brother Paul Weeks was on board the flight.

"Why not wait and get everybody on the same page so the families don't need to go through this turmoil?"

Many of the Chinese relatives have consistently questioned the official view that the plane crashed.

Some gathered outside the Malaysia Airlines offices in Beijing on Thursday to demand answers.

Among them was Dai Shuqin, sister of one of the passengers.

"France is being cautious about it, but Malaysia is desperate to put an end to this case and run away from all responsibilities," she said.

"I don't believe this latest information about the plane, they have been lying to us from the beginning," Zhang Yongli, whose daughter was on board, told AFP news agency.

"I know my daughter is out there, but they won't tell us the truth."

Liu Kun, whose younger brother was on the plane, said: "Find the people for us. We suspect that the plane wreckage could be faked."

In another development, the Malaysian transport minister said more suspected plane debris had been found on Reunion, including window panes and seat cushions.

Liow Tiong Lai said the items had been sent to French authorities to be verified.

However, French investigators quoted by AFP news agency said no new debris had been received.

Liow also said elements of the flaperon, including the paint colour, matched with maintenance records for the missing flight.

The lack of unity over whether parts of MH370 had been found has angered many of the families of those missing.

Najib's announcement came after the first day of tests on the flaperon in Toulouse.

He said investigators had "conclusively confirmed that the aircraft debris found on Reunion Island is indeed from MH370".

French prosecutor Serge Mackowiak said only there were "very strong indications" this was the case, and that confirmation would only come after further tests.

The BBC's Hugh Schofield in Paris said Mackowiak's caution did not suggest he had doubts, but that he was exercising legal caution.

Australia, which is leading the search for the plane in the southern Indian Ocean, would only say that it remained confident it was searching in the right area.

Australian Transport Minister Warren Truss said Malaysia, which is in charge of the overall investigation, had the right to express its view.

"We respect the view that they believe they have sufficient evidence to make a categorical statement of that nature,'' he said.

"The French inquiry, of course, has not been quite so conclusive."

Liow said he understood why the French team had been less categorical.

"We respect their decision to continue with their verification," he said.

"For the Malaysian team, the technical report and maintenance report that we have matched with the flaperon... the expert team strongly feel and confirm that it is MH370."

Meanwhile, Australian Prime Minister Tony Abbott said the search for the body of the plane would continue as "we owe it to the hundreds of millions of people who use our skies".

The Australian Transport Safety Bureau (ATSB) has been co-ordinating the deep-sea hunt in the southern Indian Ocean, where the plane is believed to have gone down, thousands of miles east of Reunion.