In a pre-recorded message Malaysia civil aviation chief Azharuddin Abdul Rahman said, "It is with the heaviest heart and deepest sorrow that we officially declare Malaysia Airlines Flight MH370 an accident."
All 239 passengers and crew on board are presumed to have lost their lives, he said in the message broadcast on Malaysian television.
Despite an international search operation, no trace of the Beijing-bound aircraft has been found so far.
Officials said that the recovery operation is ongoing.
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Azharuddin said that the search for the Boeing 777-200 aircraft "remains a priority."
He said that "Chicago Convention" on International Civil Aviation states that the definition of the term "accident" includes "the aircraft is missing".
"It also states that 'an aircraft is considered to be missing when the official search has been terminated and the wreckage has not been located.'"
Azharuddin said the investigation by the safety team and Malaysian police were ongoing, but both were limited by the lack of physical evidence at this time, particularly the flight recorders.
The announcement means that the next of kin can move ahead with claims of compensation and insurance.
Indians on board the plane were Chetna Kolekar, Swanand Kolekar, Suresh Kolekar, Chandrika Sharma and Prahlad Shirsatha.