Najib's government has so far been tight-lipped about its investigation into the disappearance of the Malaysia Airlines jet, adding to the anger and frustration among relatives of the 239 people, including five Indians, aboard the plane.
The Boeing 777-200 mysteriously vanished on March 8 en route from Kuala Lumpur to Beijing and is now believed to have crashed in the Indian Ocean, where an Australian-led effort is under way to recover its black boxes and wreckage.
Quizzed why no planes were sent up to investigate when Malaysian military radar spotted that a plane did a turn back, Najib said: "No, because - simply because it was deemed not to be hostile."
"Now, the military radar, the primary radar, has some capability. It tracked a - an aircraft, which did a turn back. But they were not sure - exactly sure whether it was MH370. What they were sure of was that the aircraft was not deemed to be hostile," Najib told CNN in an interview.
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"They were not sure. But it behaved like a commercial airline," he said.
The premier said he thought it was "bizarre scenario" when he was told by experts that Flight MH370 flew in another direction instead of heading towards Beijing.
"To be honest, I found it hard to believe to begin with, because how could a plane that was supposed to be heading towards Beijing - they could decide that the plane ended halfway towards Antarctica. It's a bizarre scenario, which none of us could have contemplated," he said.
Asked if he was prepared now to say the plane and its passengers have been lost, nearly 50 days after the plane disappeared, Najib said: "On the balance of the evidence, it would be hard to imagine otherwise...," he said.
"At some point in time, I would be," he said.
"Right now, I think I need to take into account the feelings of the next of kin. And some of them have said publicly that they're not willing to accept it until they find hard evidence," he said.
The mystery of the missing plane continued to baffle aviation and security authorities who have so far not succeeded in tracking the aircraft despite deploying hi-tech radar and other gadgets.