Some 20 Chinese family members gathered outside the Malaysia Airlines (MAS) office in Beijing on Thursday after plane debris found on a French island in the Indian Ocean was declared to be part of missing flight MH370 by Malaysian Prime Minister Najib Razak late on Wednesday night.

Amid police presence, they wore signs that said "Malaysia hide truth" and "We'll never give up".

Many next of kin are unconvinced that the plane part is part of MH370. They believe instead that the jetliner is embroiled in a "political conspiracy" and that Malaysia is hiding the truth.

"We believe our loved ones are still alive. The plane might be lost but they are still alive. We believe the (Communist) party will help us," one family member said as she knelt outside the building's lobby asking the media to help reveal the truth.

Madam Dai Shuqin, 62, whose sister Dai Shuling was on board, told The Straits Times that they are also at the office to demand that MAS helps arrange a trip to the French Island of Reunion.

"If they want to claim that the plane has been found, they better show it and prove it to us, if not we'll never believe them," she said.

The relatives were also livid that MAS did not inform them about the latest development beforehand. They had found out about Datuk Seri Najib's announcement from the news.

No MAS employee has been in contact with them since the announcement, they said.

The aircraft vanished on March 8 last year while en route from Kuala Lumpur to Beijing. Of its 239 passengers on board, 153 were Chinese nationals.