The search for a Malaysia Airlines plane which was presumed to have crashed more than 16 months ago has already cost about €100 million (S$150 million), or more than €200,000 a day.

The search expenses are being borne mainly by Australia, Malaysia and China, whose citizens accounted for more than six in 10 passengers on board the ill-fated flight.

They do not include indirect costs such as assistance rendered to families of the passengers, said Mr Raymond Benjamin, secretary-general of the International Civil Aviation Organisation (ICAO), a United Nations arm which oversees global commercial aviation.

Flight MH370 went missing about an hour after it left Kuala Lumpur for Beijing on March 8 last year, with 239 people on board.

Despite intensive search efforts in the Indian Ocean into which the plane is believed to have plunged, no physical trace of the Boeing 777 jet has been found.

In an exclusive interview with The Straits Times on Friday, Mr Benjamin said the aircraft, which allegedly flew for hours without any contact with the ground before it ended its journey, had to be found if the mystery of its disappearance was ever to be resolved.

"We don't know what exactly happened in the cockpit so we don't know if it was a security issue or a safety issue. We have never had a situation where an aircraft flew for seven hours before crashing."

Mr Benjamin, who was in Singapore for an aviation cyber-security conference organised by the Transport Ministry, said it was important to know the cause of an accident to prevent a repeat. "Without the aircraft wreckage, we don't know."

But with little clue as to its location and the consistently rough seas and high winds in the areas being searched, the odds have been stacked against search teams.

As the plane is thought to be in Australia's search and rescue zone, it agreed soon after the incident to lead the hunt for the plane.

But the country, which lost six nationals in the tragedy, is starting to buckle under the financial strain.

The cost to date is already more than three times the amount that was spent, over two years, in the hunt for Air France's Flight AF447, which crashed into the Atlantic Ocean in 2009.

Australia, which has forked out more than any other country, has now asked for support from other Indian Ocean coastal nations and gone to the ICAO for clarity on the country that should take ultimate responsibility for such deep-sea search operations.

Singapore, deploying air and naval resources, was one of many nations involved in the initial phase of the search.

National University of Singapore professor of aviation law Alan Tan said there are no international agreements on the country or party that should bear the costs of an aircraft search.

The Chicago Convention, which regulates global aviation, says that when a plane crashes into international waters, the country of registry - in this case, Malaysia - takes the lead and can request the nearest state to assist.

Prof Tan said: "Australia's financial commitment is voluntary and would have been offered as a matter of diplomatic relations.

"Australia is a wealthy country and that is partly why the search has gone on for so long. But even then, there is clearly a limit to how long the Australians can continue to conduct and finance the search."

In the case of the Air France crash, the aircraft manufacturer, Airbus, contributed to the costs of the search.

"But that itself was highly unusual, and in a case like MH370, where it is still a mystery whether the plane's mechanics had anything to do with its loss, it would be difficult to force the manufacturers to pay search costs," Prof Tan said.

"The reality, then, is that the question of who foots the bill will probably be left to individual states and how they negotiate this diplomatically," he said.

Even as the ICAO has said it would review global conventions following Australia's concerns, Mr Benjamin does not foresee that the hunt will be called off.

"I do not believe the search will be stopped. It will continue for some time," he said, adding it was the responsibility of Malaysia, China and Australia to see to it that the plane is found.

Mr Benjamin said: "For the moment, there is a strong will on the part of all of these parties to continue and, in particular, China and Malaysia."