KUALA LUMPUR: Malaysia's premier pledged to release a report on flight MH370's disappearance as passengers' families protested Friday outside the country's embassy in Beijing, venting anger at the agonising information vacuum surrounding the plane.

Prime Minister Najib Razak, whose government has faced wide questioning over its transparency on MH370, promised that a preliminary report submitted to the UN's aviation body would be released publicly.

“In the name of transparency, we will release the report next week,” he told CNN in an interview aired late Thursday.

That wasn't soon enough for dozens of Chinese relatives who held an overnight protest outside the Malaysian embassy in Beijing, according to a spokesman for relatives.

Many family members, especially those in China — two-thirds of the 239 people aboard the Malaysia Airlines plane were Chinese — have for weeks bitterly accused Malaysia of a secretive and incompetent MH370 response.

Tensions boiled over at a briefing Thursday at a hotel where relatives are staying, after airline representatives said a Malaysian embassy official would not arrive to answer their often extremely combative questions.

“We want somebody from the embassy to come out and tell us why they didn't come,” said relative Steven Wang.

He said about 100 people had waited outside the mission overnight.

Police fanned out around the embassy Friday morning.

Dozens of relatives had staged a noisy protest last month at the embassy — apparently sanctioned by Chinese authorities, who cleared streets for their approach — decrying Malaysian authorities and the national airline as “murderers”.

The Boeing 777 vanished on March 8 en route from Kuala Lumpur to Beijing and is now believed to have crashed in the southern Indian Ocean, where an Australian-led search is under way.

But a difficult underwater search of the suspected crash site, using an unmanned mini-submarine equipped with a sonar device, was nearing completion with no trace of the plane found.

Search 95 per cent complete

“Bluefin-21 has now completed approximately 95 per cent of the focused underwater search area. No contacts of interest have been found to date,” said a statement from the search headquarters in Perth.

The UN's International Civil Aviation Organisation (ICAO) requires countries to submit within 30 days a factual account of what is known so far in any air crash.

A Malaysian official had said Wednesday it was uncertain whether the government would release the report.

But Najib confirmed Malaysia would release it publicly after an “internal investigation team” examined it.

Asked on CNN whether that indicated it contained embarrassing revelations, Najib replied, “No, I don't think so.”

Anthony Brickhouse, a member of the International Society of Air Safety Investigators, said the report was unlikely to contain anything startling.

“This preliminary report is really just a run-down of what you know so far.

And in this case, not much is known anyway,” he said.

Malaysia has pledged that any data eventually recovered from the plane's “black box” will be publicly released.

It has said it is assembling what officials insist will be an independent international team operating under ICAO guidelines to conduct a comprehensive probe.

Australian and Malaysia authorities are mulling what to do next in the ocean search if the Bluefin-21 fails to find wreckage.

But they insist the search — estimated to have cost at least $100 million and counting — will go on, possibly using other assets including more powerful sonar devices.

Najib stressed that his government was not yet prepared to declare MH370's passengers dead, while saying, “it is hard to imagine otherwise.”

A relatives' organisation this week denounced a Malaysian official's suggestion that death certificates could soon be issued. The outraged families said that would be premature in the absence of any proof of what happened.