The search for missing Malaysia Airlines Flight MH370 is set to be the most expensive in aviation history, analysts say, as efforts to find the aircraft deep under the Indian Ocean show no signs of slowing.

The Boeing 777 vanished on March 8 with 239 people on board, after veering dramatically off course en route from Kuala Lumpur to Beijing and is believed to have crashed in the sea off Australia.

Australia, which is leading the search in a remote patch of water described as "unknown to man", has not put a figure on spending, but Malaysia has warned that costs will be "huge".

"When we look at salvaging (wreckage) at a depth of 4.5 kilometres (2.8 miles), no military out there has the capacity to do it," Transport and Defence Minister Hishammuddin Hussein said Thursday.

"We have to look at contractors, and the cost of that will be huge."

Ravikumar Madavaram, an aviation expert at Frost & Sullivan Asia Pacific, said Malaysia, Australia and China, which had the most nationals onboard the flight, were the biggest spenders and estimated the total cost up to now at about US$100 million (72 million euros).

"It's difficult to say how much is the cost of this operation ... but, yes, this is definitely the biggest operation ever (in aviation history).

"In terms of costs this would be the highest," he told AFP.

- Hopes rest on submersible -

In the first month of the search -- in which the South China Sea and Malacca Strait were also scoured by the US, Malaysia, Singapore and Vietnam -- the Pentagon said the United States military had committed US$7.3 million to efforts to find the plane.

Meanwhile the Indian Ocean search, in which assets have also been deployed by Australia, Britain, China, South Korea, Japan and New Zealand, has failed to find anything conclusive.

Hopes rest on a torpedo-shaped US Navy submersible, which is searching the ocean floor at depths of more than 4,500 metres (15,000 feet) in the vicinity of where four signals believed to have come from black box recorders were detected.

David Gleave, an aviation safety researcher at Britain's Loughborough University, said the costs "will be of the order of a hundred million dollars by the time we're finished, if we have found it (the plane) now".

But he said the longer it took to find any wreckage, the more costs would mount because scanning the vast ocean floor "will take a lot of money because you can only search about 50 square kilometres (19 square miles) a day".

Salvaging anything would also depend on how deep the ocean is at the crash point and how dispersed the wreckage, with weather and politics also complicating factors, he said.

The fate of MH370 has drawn parallels with the hunt for Air France Flight 447 which plunged into the Atlantic in 2009.

The two-year operation to recover its black box, which involved assets from France, Brazil and the US, has been estimated to have cost 80-100 million euros, according to figures cited by France's Investigation and Analysis Bureau (BEA).

- 'One of the most difficult searches ever' -

Australia's Joint Agency Coordination Centre says its main focus is still on finding flight MH370.

"It is one of the most difficult searches ever undertaken and could take some time," JACC said in a statement to AFP.

"The cost of the search is significant. The exact figure has not yet been calculated.

"The cost is being shared by our international partners who have contributed their people and military and civilian assets to help with the search."

As the search continues, all international partners are meeting their own costs. But governments and militaries will need to consider the broader cost implications of the search down the track, said Kym Bergmann, editor of Asia-Pacific Defence Reporter.

"I don't think that the Australians would be getting any change at all out of Aus$1 million day," he told AFP.

Bergman said it would likely be the most expensive aviation search given how long it had already dragged on.

"It must be starting to worry military planners," he said, adding that any decision to scale back would cause heartache to the families involved.

Malaysia-based Madavaram agreed, saying at present it was still "politically insensitive" to cut spending.

"I think they will continue one or two months irrespective of the costs," he said. "But then if nothing is found, it will become a wild goose chase, and people will start questioning it."