"With deep sadness and regret that I must inform you that, according to this new data, flight MH370 ended in the southern Indian Ocean," a grim-faced Najib told a specially convened press conference.
The announcement came on the fifth day of an international search effort in the southern Indian Ocean, with Australian and Chinese planes reporting spotting of several floating objects, about 2,500 km west of Perth.
There is no official word yet on the wreckage of the Boeing 777-200 that went missing on March 8. Najib said he will hold a press conference tomorrow, indicating that he will then come out with more information on the aircraft.
Based on new analysis UK's Air Accidents Investigation Branch (AAIB) and Inmarsat, the British company that provided satellite data, "we have concluded that MH370 flew along the southern corridor, and that its last position was in the middle of the Indian Ocean, west of Perth," he said.
"This is a remote location, far from any possible landing sites."
"We share this information out of a commitment to openness and respect for the families, two principles guiding this investigation," he said.
He said Malaysia Airlines officials have already spoken to the families of the passengers and crew to inform them of the new development.
"I urge the media to respect their privacy, and to allow them the space they need at this difficult time."
His announcement came 17 days after the Beijing-bound plane with 239 people, including five Indians, on board disappeared mysteriously from radar screens, one hour after taking off from Kuala Lumpur.
The list of passengers on board included 154 Chinese, 38 Malaysians, 7 Indonesians, 6 Australians, 5 Indians, 4 Americans and 2 Canadians.
Indians, including three from one family, were identified as Chetna Kolekar, 55, Swanand Kolekar, 23, Vinod Kolekar, 59, Chandrika Sharma, 51, and Kranti Shirsatha, 44.
Meanwhile, an Australian ship was trying to retrieve objects located in the area earlier in the day.
Two objects -- the first grey or green and circular and the second orange and rectangular -- located by an Australian P3 Orion aircraft in the area, Australian Premier Tony Abbott said.
A Chinese Ilyushin-76 plane reported spotting "white and square" objects in the same location.
Malaysia Airlines in a statement said, "We humbly offer our sincere thoughts, prayers and condolences to everyone affected by this tragedy."
"On behalf of all of us at Malaysia Airlines and all Malaysians, our prayers go out to all the loved ones of the 226 passengers and of our 13 friends and colleagues at this enormously painful time."
"We know there are no words that we or anyone else can say which can ease your pain. We will continue to provide assistance and support to you, as we have done since MH370 first disappeared in the early hours of 8 March, while flying from Kuala Lumpur to Beijing," it said.
The firm said the ongoing multinational search operation will continue, as authorities 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 said.
Authorities believe, based on radar and satellite data, that the plane was deliberately taken off-course after the communication system shutdown by someone on board.
Based on information, the search has been in two distinct corridors - one stretching to the north-west of the last known location in the Malacca Straits and one to the south-west.
Since none of the countries on the northern corridor have reported any radar contact with the missing plane or satellite images, the search for possible debris has been concentrated in the southern Indian Ocean.
Police have also interviewed more than 100 people, including the relatives of the pilot and the co-pilot to find out what caused the tragedy as they are looking at all possibilities, including hijacking, sabotage and terrorism.