Najib said that the flight route was declared safe by International Civil Aviation Organisation and International Air Transport Association (IATA) stated that the airspace MH17 was traversing was not subjected to restrictions, the Malaysian Star reported.
He said that Ukrainian authorities believed that the plane was shot down, but it is yet to be verified.
"If it is confirmed that the plane is shot down, then the perpetrators must be brought to justice," Najib said at a press conference in Sepang.
The flight took off from Amsterdam's Schiphol airport
Najib said that he has spoken to Ukraine President Petro Poroshenko who pledged a full, thorough investigation.
He said that the Ukrainian president had confirmed negotiations with rebels for the establishment of a humanitarian corridor.
Najib also said that he had spoken to US President Obama and they had agreed that an international team should have full access to the crash site, and that no one was to move any debris including the black box.
"This is a tragic day in what has already been a tragic year for Malaysia. The flight's passengers were from many nations but we are all united in grief," said Najib.
The Kuala Lumpur-bound Malaysia Airlines MH17 passenger plane crashed Thursday near the Russian border, with all the 298 passengers and crew members on board reportedly killed.
The flight took off from Amsterdam's Schiphol airport.