An interim report of the investigation released on Sunday said there were "no behavioural signs of social isolation, change in interests or habits or interest, self-neglect, drug or alcohol abuse of the captain, first officer and the cabin crew."

The report said the ability of the plane's captain Zaharie Ahmad Shah to handle stress at work and home was good and he had no history of apathy, anxiety or irritability.

The report said there was also nothing to suggest anything sinister in the behaviour of other more junior pilot Fariq Abdul Hamid.

The possibility that one of the pilots or crew hijacked the plane has been one of the most plausible theories about what happened to the missing flight.

Fingers have been pointed at Captain Zaharie who according to some reports had problems with his marriage and was upset with Malaysia's government over its jailing of opposition leader Anwar Ibrahim.

But the report provides no evidence to support a so-called "rogue pilot" theory, saying Captain Zaharie held bank accounts with his wife, owned two houses and had not taken out an insurance policy.

The report also revealed the battery of an underwater locator beacon had expired more than a year before the plane vanished.

Apart from that anomaly, the 584-page report devoted a large chunk to describing the complete normality of the flight, shedding little light on aviation's biggest mystery.

The expired battery would have had lesser chance of locating the aircraft in the Indian Ocean, where it is believed to have crashed, even if searchers were in its vicinity.

The report said there was no indications of problems with the plane's engines, electrical, mechanical or oxygen systems.

But it revealed there was massive confusion among civilian and military aviation authorities in the hours after the plane changed course when it was still in the air.

Malaysia Airlines said on Sunday investigators have no idea what happened to the plane.

"We are no closer to finding a resolution," airline chairman Md Nor Yusof told a gathering of the company's staff to mark the one year anniversary of the disappearance.

"We are still asking and seeking for answers to what happened to MH370," he said, adding the tragedy and chain of events afterwards were unprecedented.

Malaysia's Prime Minister Najib Razak issued a statement saying "together with our international partners, we have followed the little evidence that exists," as families of those on board vowed never to give up on the search for wreckage and answers.

Mr Najib praised dozens of experts who have been working since April last year to try establish why one of the world's most sophisticated airliners abruptly lost communication and turned around from its scheduled flight from Kuala Lumpur to Beijing.

The plane flew on for thousands of kilometres in the early morning darkness and, according to available data, crashed into a vast expanse of the southern Indian Ocean west of Perth.

"The disappearance of MH370 is without a precedent and so too is the search – by far the most complex and technically challenging in aviation history," Mr Najib said.

"No words can describe the pain the families of those on board are going through…the lack of answers and definitive proof – such as aircraft wreckage – has made this difficult to bear," he said.

Mr Najib said Malaysia remains committed to finding the plane while China's foreign minister Wang Yi promised relatives the search would not stop.

A majority of the passengers were Chinese.

Prime Minister Tony Abbott also said the search would be extended beyond a four-ship search of 60,000 square kilometres of the ocean floor that is due to conclude in May.

"We are reasonably optimistic of success but if we don't succeed in this search there is another search that we intend to make because we owe it to the families of the dead, we owe it to the travelling public, to do whatever we reasonably can  to the resolve of this mystery," he said.

The search, costing more than $120 million, is the costliest in aviation history.

In late January, Malaysia's government formally declared the flight an accident and said all 239 people were dead, opening the way for Malaysia Airlines to pay compensation to relatives of the victims.

But the statement angered many relatives who said it was premature without any answers or physical evidence of the crash.

A group of relatives called Voice 370 held a Day of Remembrance event at a mall in Kuala Lumpur on Sunday.

About 30 relatives of the Chinese passengers on board MH370 gathered at a Buddhist temple in central Beijing to mark the anniversary of the plane's disappearance.

With a strong police presence watching on, the relatives held signs and wore white t-shirts which said "Never Give Up, Search On". The next of kin said they wanted to pray for their loved ones while maintaining pressure on Malaysian authorities, who they believe are concealing information.

Since the plane's disappearance, relatives say that Chinese security officials have tightened their watch over them, especially after some began criticising the Chinese government's response to the incident.

"We want to show our determination," said Jiang Hui, whose mother was aboard the flight.  "We are here to pray for our loved ones and we hope they can come back and the truth will come out as soon as possible."

With Philip Wen