It comes amid a deepening diplomatic row over the February killing of North Korean leader Kim Jong Un's half-brother at Kuala Lumpur International Airport 2.
"What we are facing now is the result of their action in assassinating their own citizen in Malaysia, on Malaysian soil, using a strictly banned chemical weapon," Najib told state media agency Bernama Wednesday.
A press official for Najib confirmed the comments to CNN. North Korea has repeatedly denied any involvement in Kim Jong Nam's murder.
Kim died on February 13, fewer than 20 minutes after two women wiped his face with the highly toxic VX nerve agent
, according to Malaysian police.
The ensuing investigation into Kim's murder has severely damaged diplomatic relations between the two countries.
A new low was reached Tuesday when North Korea barred Malaysian nationals inside the country from leaving. Malaysia swiftly followed suit.
Until Wednesday, only South Korea's intelligence agency had publicly linked North Korea to the killing, which it said was carried out on the orders of leader Kim Jong Un.
Najib's comments could complicate efforts to secure the return of 11 Malaysian nationals held in North Korea.
They include four embassy staff members, their families and two UN employees -- a man and a woman who work for the World Food Programme, according to the UN.
Malaysia says their 11 citizens are free to live their daily lives but are not free to leave.
Malaysia believes three people wanted for questioning in connection to Kim's murder are holed up in the North Korean embassy in Kuala Lumpur.
Police say North Korea is not complying with its request to question them.
Only two people, a woman from Vietnam and another from Indonesia, have been formally charged with Kim's murder.
Authorities from both countries have said the women believed they were participating in a prank TV show, but Malaysian authorities do not believe that's true.
A third North Korean who was in custody was released after investigators determined there wasn't enough evidence to charge him. He was then deported back to North Korea.
Malaysia is seeking four other suspects in relation to the killing, all North Korean men who are believed to be back in Pyongyang.
Police said Tuesday North Korea had not responded to requests to hand them over. They have requested assistance from Interpol, the international police service.