Malaysian Prime Minister Najib Razak vowed Thursday "we will not rest" until the fate of Flight MH370 is known, as Australia called it "the most difficult search in human history".
Najib toured the military base in Perth being used as a staging post in the hunt for the Malaysia Airlines plane carrying 239 people that is believed to have crashed in the Indian Ocean, and pledged never to give up looking for answers.
"We want to find answers. We want to provide comfort to the families and we will not rest until answers are indeed found," he said, as he thanked those involved in the eight-nation search.
Despite extensive scouring of the remote southern Indian Ocean off Perth, no debris that would indicate a crash site has so far been found.
Najib admitted the exhaustive hunt for the Boeing 777 that vanished on March 8 was a "gargantuan task" but said he was confident the baffling disappearance would be solved.
"I am very confident with the level of professionalism... that indeed in due time we will provide a closure to this event, on this tragedy," he said.
Kuala Lumpur's handling of the crisis has been widely criticised, especially by distraught relatives of the 153 Chinese nationals aboard.
Adding to the frustration for families affected, Malaysian police chief Khalid Abu Bakar said Wednesday a criminal investigation into what caused the flight to veer from its intended route between Kuala Lumpur and Beijing had so far been inconclusive.
In contrast, Australia's mobilisation since it was handed increased responsibility in the search effort has been praised.
Australia has far more experience than Malaysia of rescue operations, routinely monitoring huge tracts of ocean, but Prime Minister Tony Abbott said the current search was the toughest ever.
"Every day, working on the basis of just small pieces of information, we are putting the jigsaw together. And every day we have a higher degree of confidence that we know more about what happened to this ill-fated flight," he said.
"It is a very difficult search, the most difficult in human history, but as far as Australia is concerned we are throwing everything we have at it."
- International cooperation -
Since the plane disappeared nearly a month ago, eight nations, many of whom do not normally work together, have rallied to help track down clues to one of the greatest aviation mysteries the world has seen.
Both Najib and Abbott hailed the "truly remarkable" international cooperation, which involves Australia, Britain, China, Japan, New Zealand, Malaysia, South Korea and the United States.
"To have the military forces of so many countries working together for our common humanity shows what we can do," said Abbott.
"It shows what we can do and if anyone would ever be unhappy or distraught about the prospects for international peace and harmony this operation is a marvellous antidote to pessimism."
A British nuclear submarine with underwater search capabilities reinforced the hunt Wednesday, joining planes and ships scouring 237,000 square kilometres (91,500 square miles) of desolate seas, but they again reported seeing nothing of interest.
Australia's Joint Agency Coordination Centre said eight planes and nine ships were involved Thursday as they further refined the area they are looking in to 1,680 kilometres west north-west of Perth.
Australia's Ocean Shield naval vessel, which is fitted with a US-supplied "black box" detector, is due to arrive in the area Friday, but it it faces a daunting task and the search zone needs to be narrowed for it to have any hope of finding evidence of the doomed jet.
- Passengers cleared of suspicion -
Malaysian police chief Khalid on Wednesday pleaded for more time to work out what happened to NH370, with news agency Bernama quoting him as saying all passengers had been "cleared" of suspicion.
Police 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.
But authorities remain unclear on how and why it vanished, 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 -- usually lasts only about 30 days, with time fast running out.
On Wednesday, Malaysian officials sought to explain to sceptical 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.