Malaysia's police chief says at least two suspects in the nerve agent assassination of Kim Jong-nam are holed up in North Korea's barricaded embassy in Kuala Lumpur and police would wait as long as necessary to arrest them.

"If it takes five years we will wait outside," declared Khalid Abu Bakar, adding "definitely somebody will come out."

The United Nations has called for calm after North Korea barred Malaysians from leaving the country, prompting Malaysia to ban North Koreans leaving Malaysia in a rarely seen diplomatic meltdown over Mr Kim's killing at Kuala Lumpur airport on February 13.

Malaysia's prime minister Najib Razak strongly condemned North Korea's ban on Tuesday, saying it is "effectively holding our citizens hostage."