Sydney, Australia: The massive underwater search for missing Malaysia Airlines flight MH370 was called off Tuesday, leaving unsolved one of the most enduring mysteries of the aviation age.

Nearly three years after the airliner vanished, distraught relatives refused to accept the idea that the 239 passengers and crew might now never be found after the failure of one of the most expensive undersea operations ever.

"Despite every effort using the best science available, cutting edge technology, as well as modelling and advice from highly skilled professionals who are the best in their field, unfortunately, the search has not been able to locate the aircraft," said a statement from Australia, China and Malaysia.

"Accordingly, the underwater search for MH370 has been suspended." 

The Malaysia Airlines jet disappeared en route from Kuala Lumpur to Beijing on March 8, 2014.

An initial search focused on an area east of peninsular Malaysia, but attention soon shifted to the west when it emerged the plane had changed course and headed into the Indian Ocean -- just as its communications equipment had been switched off.

The information spawned speculation that the plane had been hijacked or rerouted, but little supporting evidence could be produced and conspiracy theories abounded.

Investigators later focused their search on a 120,000 square kilometre (46,000 square mile) area to the west of Australia.

The area was determined based on scant clues available from satellite "pings" and calculations of how much fuel was on board, which suggested the plane had ditched in the southern Indian Ocean.

Deep water search specialists scoured the ocean floor at depths of up to several thousand metres (feet) for almost two years before declaring they had done as much as they could.

Malaysia Airlines hailed the search effort as "thorough and comprehensive", adding it was hopeful "new and significant information will come to light and the aircraft would eventually be located".

- Unprecedented scale -

Relatives lashed out at the announcement, with campaign group Voice370 calling on authorities to prolong the hunt, which has cost upwards of $135 million.

"In our view, extending the search to the new area defined by the experts is an inescapable duty owed to the flying public in the interest of aviation safety," Voice370 said in a statement.

"Commercial planes cannot just be allowed to disappear without a trace."

Many relatives have repeatedly complained about the lack of a coordinated search in the western Indian Ocean and along the African coast, where three pieces of debris confirmed to have come from the stricken plane have been found.

Search coordinators countered that oceanic drift patterns were consistent with debris making its way from the presumed crash site to the western fringes of the Indian Ocean.

The search for MH370 was on an unprecedented scale and in one of the world's remotest locations, where winds tear up north from Antarctica whipping up mountainous seas.

The lack of a final resting place for MH370 has spawned numerous ideas, including that it was a hijacking or terror plot. The jet's captain Zaharie Ahmad Shah also came under scrutiny, although investigations on him have turned up nothing.

A. Amirtham, 62, whose only son S. Puspanathan was on board MH370, spoke of the pain she and her husband G. Subramaniam have endured over the years.

"Deep down in my heart, I believe he is alive," she told AFP in Kuala Lumpur.

"How can they stop the search when they have not found the plane? I am sad and confused because I just do not know if my son is dead or alive."

China's Jiang Hui, whose mother Jiang Cuiyun was also on MH370, said he was disappointed and angry that the search was being halted now.

"We believe that the main reason for stopping the investigation is a lack of funds," he told AFP.

"It is not because human technology cannot get results. It is not because each country has done their best."

- 'Backbone' -

The end of the search, which had been a joint operation by the governments of Australia, Malaysia and China, was flagged months ago, with authorities saying in the absence of significant new information there was nothing more to go on.

AirlineRatings.com editor Geoffrey Thomas said it was possible the search could continue if it was privately funded, as he warned that the failure to extend the search could fuel conspiracy theories.

"It's highly likely that someone in the world will see the value in finding this aeroplane to bring closure for the relatives," he told AFP.

"But also from the aviation point of view, of finding out what happened to the aeroplane because the 777 is the backbone of the world's fleet."