The head of the International Civil Aviation Organization has warned the truth behind the disappearance of the missing Malaysia Airlines flight MH370 will never be known until the wreckage is found.

The Boeing 777 has now been missing since 8 March 2014 after it vanished from radar on its way to Beijing from Kuala Lumpur.

No sign of the plane has ever been discovered. But according to data, it veered off course and flew for seven hours before crashing into the Indian Ocean.

Countless theories have since emerged attempting to explain its disappearance. But ICAO secretary-general Raymond Benjamin said without finding the plane, we will never know what caused it to crash.

"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," he told Straits Times.

"We have never had a situation where an aircraft flew for seven hours before crashing. Without the aircraft wreckage, we don't know."

In May, Australia said it will contribute an extra £40.48m ($63.4m) over the next two years to locate MH370 if the plane is not found during the current official search.

Malaysia, China and Australia will continue to take responsibility for the search. So far, the 16-month investigation is estimated to have cost £71m.

The sonar equipement used in the search, led by Dutch company Fugro, has come under criticism for missing signs of the wreckage entirely because the quality of its images get worse the further it travels, according to reports. Those claims have been refuted by its director, Paul Kennedy, who said the 75kHz sonar devices are a well-established technology.