A British nuclear sub with underwater search capabilities reinforced the Indian Ocean hunt for flight MH370 on Wednesday, but Malaysian police warned the cause of the jet's mysterious disappearance may never be known.

Despite extensive scouring of the remote waters southwest of Perth where Malaysia believes the jet went down, ships and planes have found no debris that would indicate a crash site.

Malaysian police chief Khalid Abu Bakar said a three-week-old criminal investigation into what caused the flight with 239 people aboard to veer far from its intended route between Kuala Lumpur and Beijing had so far been inconclusive.

The sober assessment will add to the frustrations of anxious family members of the missing passengers, who have denounced Malaysia's handling of the crisis.

"Give us more time," Khalid told reporters in Kuala Lumpur, according to Dow Jones Newswires. "We may not even know the real cause of this incident."

Ten planes and nine ships resumed the hunt Wednesday although authorities warned clouds, fog and isolated thunderstorms would reduce visibility.

Britain's Royal Navy said the first submarine in the operation -- HMS Tireless -- has arrived in the area and "with her advanced underwater search capability will be able to contribute to the attempts to locate the missing plane".

Britain's HMS Echo is also due in the search zone shortly to assist Australia's Ocean Shield naval vessel, which is fitted with a US-supplied "black box" detector and is expected to arrive on Friday.

-- Black box deadline looms --

Malaysian news agency Bernama quoted police chief Khalid saying all MH370 passengers had been "cleared" of suspicion.

Police also are investigating the backgrounds of the plane's 12 crew, as well as ground staff and flight engineers, for signs of a hijack or sabotage plot.

More than three weeks after the flight's March 8 disappearance, authorities remain baffled as to how and why it happened, and warn that unless the black box is found, the mystery may never be solved.

The battery-powered signal from MH370's black box -- which records flight data and cockpit voice communications that could hold clues in the mystery -- usually lasts only about 30 days, with time fast running out to find it.

Australia has warned against expectations of quick success in the difficult task of recovering the black box from the deep and vast seas.

Retired Australian air chief marshal Angus Houston, who is heading a new coordination centre in Perth, reiterated Wednesday that the odds were stacked against them.

"The reality is it's the most complex and challenging search and recovery operation I've ever seen," he told national radio.

-- Malaysian leader heads to Perth -

Malaysian Prime Minister Najib is due to arrive late Wednesday in Perth.

On Thursday, he will tour the air base being used as a staging post for the search and meet with his Australian counterpart Tony Abbott.

Malaysia's handling of the crisis, marked by piecemeal and occasionally contradictory information, has been widely questioned, especially by distraught relatives of the 153 Chinese nationals aboard.

Many have alleged incompetence or even a cover-up by Malaysia, straining ties between the two countries, although the rhetoric has eased in recent days.

In a briefing on Wednesday, Malaysian officials sought to explain to skeptical relatives their conclusion that the plane went down in the Indian Ocean.

That determination, based on complex satellite data, has outraged some families who say wreckage must first be recovered.

The closed-door briefing outside Kuala Lumpur included technical experts from China and Malaysia, a government statement said, and was telecast to other relatives in Beijing.

"Throughout the process, I want to assure you that we have done everything in our power to locate MH370. We have spared no effort and expense," civil aviation chief Azharuddin Abdul Rahman told relatives in his opening statement.

But Steven Wang, a relative in Beijing, said the briefing did not convince him.

"I don't think that anything was explained to us... There are all kinds of possibilities," he said.

On Tuesday, Malaysia Airlines revealed the full radio communications with the pilots of MH370, but the routine exchanges contained nothing unusual and shed no light on the theory of possible pilot involvement.

The conversations with air traffic controllers had been the subject of much speculation as suspicions have focused on whether one or both of the pilots deliberately diverted the plane.

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