KUALA LUMPUR (AFP) - Malaysia on Thursday formally declared missing flight MH370 an "accident" and its passengers and crew presumed dead, a step that opens the door for compensation payments but which was met with angry skepticism by distraught relatives.

"It is therefore, with the heaviest heart and deepest sorrow that, on behalf of the government of Malaysia, we officially declare Malaysia Airlines flight MH370 an accident," civil aviation chief Azharuddin Abdul Rahman said in a televised announcement.

"All 239 of the passengers and crew on board MH370 are presumed to have lost their lives," he added.

Many desperate MH370 relatives have continued to insist that the plane may have landed safely somewhere, and Malaysian authorities and the airline had until now refrained from drawing firm conclusions about its fate.

But Thursday's announcement essentially declared that the plane had crashed somewhere after its disappearance last March 8.

Datuk Azharuddin acknowledged the news was "very difficult" for next of kin to digest, but added "it is nonetheless important that families try to resume normal lives".

He also said the airline was "ready to proceed immediately with the compensation process," once families are ready. Malaysia Airlines officials were not immediately available for comment.

But Ms Sarah Bajc, whose partner, Mr Philip Wood, was on board the plane, was one of several relatives who poured scorn on the announcement.

"I think they are lying," she said.

"It could very well be that the plane crashed. But there is no evidence, and until there is evidence we just can't believe them," she said, referring to the Malaysian government and flag carrier. "It is impossible to bring any closure until we have proof."

Many relatives accuse the Malaysian government and its flag carrier of a chaotic and bungled response to the plane's initial diversion, which allowed the jet to disappear, and a subsequent cover-up. Those charges are strenuously denied.

The plane vanished en route from Kuala Lumpur to Beijing in what remains one of history's great aviation mysteries.

Malaysian authorities say satellite data indicates the plane inexplicably detoured to the remote southern Indian Ocean, which they suspect was due to "deliberate" action onboard.

But no evidence has turned up despite an ongoing Australian-led search of the supposed crash region - the most expensive search and rescue operation in history.

More than two-thirds of the passengers were Chinese nationals, and their angry accusations that Malaysia had concealed information drew China's government into the fray, straining bilateral ties when Beijing criticised the disaster response.

Speaking after Malaysia's announcement, Chinese foreign ministry spokesman Hua Chunying said Beijing was "deeply saddened".

"We call on Malaysia to honour the commitments made in their statement" by providing compensation and other support to relatives in a timely manner and devoting "maximum efforts" to a transparent search and investigation effort.

Malaysia launched investigations last March, but has so far released no findings, infuriating families. Many also question the determination that the flight veered toward the Indian Ocean.

Mr Azharuddin said Malaysia would "forge ahead" with the search effort.

The government's awkward handling of Thursday's announcement also angered relatives, who have accused authorities and the airline of insensitivity and withholding information.

Mr Azharuddin was originally due to speak in a hastily scheduled news conference.

But that was abruptly abandoned after several angry MH370 relatives, including one carrying a placard demanding the return of loved ones, rushed to the venue demanding to know why they were not briefed before the announcement.

The government later said separate arrangements had made to communicate with kin, but relatives who spoke to AFP said they had not been contacted.

"There was no phone call even," said Ms Elaine Chew, whose husband, Mr Tan Size Hiang, was on the missing flight.

"I have a six-year-old daughter. I have told her that her father is missing. She wants him back for her birthday on March 14. How do I explain this to a six-year-old girl, all of a sudden?" she said.