North Korea has designated the Malaysian ambassador as "persona non grata" and is demanding the envoy leave the country within 48 hours from Sunday morning, its official KCNA news agency says.
The move is in retaliation to Malaysia's expulsion of North Korea's envoy, who questioned the impartiality of the investigation into the bizarre killing of the North Korean leader's half brother.
If North Korean agents did kill Kim Jong Nam, as US officials and South Korean intelligence suspect, the decision to assassinate him in Malaysia could cost the isolated, nuclear-armed state one of its few friends.
Malaysia's outrage over the killing of Kim Jong Nam at Kuala Lumpur International Airport was heightened by the assassins use of VX nerve agent, a toxic chemical on a UN list of weapons of mass destruction.
North Korea denies the victim of February's murder was leader Kim Jong Un's half brother, and Ambassador Kang Chol accused Malaysia's Prime Minister Najib Razak's government of colluding with external forces.
The prime minister branded Ambassador Kang "diplomatically rude" and, angered by no apology, the Malaysian government on Saturday gave him 48 hours to leave.
"They should have apologised. So based on principles, we have declared him persona non grata," Najib Razak said on Monday.
The expelled ambassador fired a parting shot at Malaysia.
"I express grave concern over the extreme measures taken by the Malaysian government, doing great harm to the bilateral relations which has a history of more than 40 years," he told reporters.
Kim Jong Nam, who had been living in Macau under Beijing's protection, spoke out against his family's dynastic control of North Korea.
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.
Until this week Malaysia was one of the few countries that North Koreans could enter visa without a visa.