Malaysia has expelled the North Korean ambassador to the country, declaring him "persona non grata" and asking the envoy to leave Malaysia within 48 hours.

The move on Saturday comes nearly three weeks after Kim Jong Nam, the estranged half-brother of North Korean leader Kim Jong Un, was murdered at Kuala Lumpur's airport with a toxic nerve agent.

US and South Korean officials have said he was killed by agents of the North Korean regime.

Kang Chol, North Korea's ambassador to Malaysia, said last month his country "cannot trust" Malaysia's handling of the probe, and also accused the country of "colluding with outside forces" in a veiled reference to bitter rival South Korea.

Malaysian foreign minister Anifah Haji Aman said in a statement on Saturday that Malaysia had demanded an apology from the ambassador for his comments, but none was forthcoming.

"Malaysia will react strongly against any insults made against it or any attempt to tarnish its reputation," Anifah said.