If North Korean agents did kill Kim Jong Nam, as US officials and South Korean intelligence suspect, the decision to assassinate him on Malaysian soil could cost the isolated, nuclear-armed state one of the few friends that it has.
Malaysia's outrage over the incident was heightened by the assassins use of VX nerve agent, a chemical so toxic that it is on a UN list of weapons of mass destruction.
North Korea has refused to accept that the victim of the February 13 murder at Kuala Lumpur International Airport was leader Kim Jong Un's half brother, and its ambassador accused Najib's government of colluding with external forces and said the investigation could not be trusted.
Najib had branded Ambassador Kang Chol "diplomatically rude" and, angered by the lack of an apology, the Malaysian government on Saturday gave him 48 hours to leave.
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"They should have apologised. So based on principles, we have declared him persona non grata," Najib told reporters at the parliament.
"This means that we are firm on the question of our honour... there is no one who can undermine us or disrupt us as they please," he said.
Asked whether Malaysia, which recalled its ambassador to Pyongyang for consultations, would be reviewing diplomatic ties with North Korea, Najib was noncommittal.
"We will see. We'll take it one step at a time."
The North Korean embassy has maintained silence since the expulsion order and Ambassador Kang has not appeared in public. He is supposed to leave Malaysia by 6 pm Monday (1000 GMT).
The two country have maintained friendly ties since the 1970s, and until this week Malaysia was one of the few countries that North Koreans could enter visa without a visa. But that privilege has been revoked.
So far, Malaysian prosecutors have charged an Indonesian woman and a Vietnamese woman for the murder.
But police have identified eight North Koreans, including a senior embassy official and state airline employee, wanted for questioning in the probe. Police say that four of the North Koreans police they want to interview left Malaysia in the hours after the murder.
Only one North Korean suspect was apprehended, but after a week in custody Ri Jong Chol was deported on Friday after being released due to insufficient evidence.
Speaking to reporters in Beijing on Saturday, Ri accused Malaysia of using coercion to try to extract a confession.
"I realised that this is a conspiracy, plot, to try to damage the status and honour of the republic," Ri said.
Kim Jong Nam, who had been living in the Chinese territory of Macau under Beijing's protection, had spoken out publicly against his family's dynastic control of isolated, nuclear-armed North Korea.
