Malaysian Prime Minister Najib Razak has informed the families of those missing on flight MH370 that the plane likely crashed in the Indian Ocean west of Perth.
Malaysia Airlines Flight MH370 vanished from civilian radar screens less than an hour after take-off from Kuala Lumpur for Beijing with 239 people on board on March 8.
No confirmed sighting of the plane has been made since, but much debris has been found in remote waters off Australia which might be part of the missing plane.
Malaysian Airlines said on Monday it was now "beyond any reasonable doubt" that missing flight MH370 had been lost and there were no survivors.
"Malaysia Airlines have already spoken to the families of the passengers and crew to inform them of this development," said the prime minister.
"For them, the past few weeks have been heartbreaking; I know this news must be harder still.
"I urge the media to respect their privacy, and to allow them the space they need at this difficult time."
In a statement released to the media and shared with the family members of passengers and crew of the missing plane, Malaysian Airlines said "the ongoing multinational search operation will continue, as we seek answers to the questions which remain".
"Alongside the search for MH370, there is an intensive investigation, which we hope will also provide answers," it added.
Stunned relatives in Beijing and Kuala Lumpur reacted with grief and anguish as their worst fears were confirmed.
In dramatic scenes in Beijing, stretcher-bearing paramedics were drafted in to tend to family members devastated by the news, which was broken to them by the airline at a hotel where they had gathered throughout the 17-day ordeal.
At least two people were borne out on stretchers, including a woman whose body was shaking, her eyes glazed and heavy with tears, as a family member held her arm.
In the lobby of a hotel outside Kuala Lumpur where relatives, including many flown in from China by Malaysia Airlines, had gathered, an elderly woman sat down hard on the floor and wept.
"He died too young, I want my son back," she cried out in Mandarin before security escorted her into an elevator.
Subramaniam Gurusamy, 60, whose 34-year-old Malaysian son Puspanathan Gurusamy was on board, had continued to hold out hope of his return throughout the agonising 17-day wait.
"I had the belief that my son would return home safely. But what can be done? This is fate. We must accept it," he told AFP, choking back tears.
Some relatives in Beijing lashed out as they left their meeting with the Malaysian flag carrier, with one man throwing punches and kicks at assembled media. One woman left the room shouting "Murderers! Murderers".