A North Korean chemist deported from Malaysia has accused police of threatening to kill his family unless he confessed to the assassination of the half-brother of North Korea's leader, calling it a plot to tarnish his country's honour.

Ri Jong Chol spoke to reporters in Beijing, China, while on his way to Pyongyang. Malaysian authorities have said there is insufficient evidence to charge him over Kim Jong Nam's killing at Kuala Lumpur airport on February 13.

Mr Ri was detained four days after the attack but police never said what they believed his role was.

Two women - one Indonesian, one Vietnamese - have been charged with murder after police said they smeared Mr Kim's face with VX, a banned nerve agent considered a weapon of mass destruction.

Mr Ri said he was not at the airport the day Mr Kim was killed but police accused him of being a mastermind and presented him with "fake evidence".

He said they showed him a picture of his wife and two children, who were staying with him in Kuala Lumpur, and threatened to kill them.

"These men kept telling me to admit to the crime, and if not, my whole family would be killed, and you too won't be safe. If you accept everything, you can live a good life in Malaysia," he said.

"This is when I realised that it was a trap ... they were plotting to tarnish my country's reputation."

Immigration director-general Mustafar Ali said Mr Ri has been banned from re-entering Malaysia.

The country's police are looking for seven other North Korean suspects, four of whom are believed to have left the country on the day of the killing.

Three others, including an official at the North Korean embassy and an employee of Air Koryo, North Korea's national carrier, are believed to still be in Malaysia.

Police on Friday issued an arrest warrant for the Air Koryo employee, Kim Uk Il, but did not say why he is a suspect . Police said he arrived in Malaysia on January 29, about two weeks before Mr Kim was killed.

The death has unleashed a diplomatic battle between Malaysia and North Korea. Malaysia said it was scraping visa-free entry for North Koreans, while the Foreign Ministry said it was "greatly concerned" about the use of the nerve agent.

Malaysia has not directly accused North Korea of being behind the killing, but the ministry statement came hours after a North Korean envoy rejected a Malaysian post-mortem finding that VX killed Mr Kim.

Ri Tong Il, a former North Korean deputy ambassador to the United Nations, said the victim probably died of a heart attack because he suffered from cardiac disease, diabetes and high blood pressure.

He said that if VX had been used, others besides Mr Kim would have been killed or injured.

The two female suspects were caught on surveillance video smearing what Malaysian authorities say was VX on his face and eyes, though both reportedly said they were duped into thinking they were playing a harmless prank.

Mr Kim died within 20 minutes, authorities say. No bystanders reported falling ill.

Police said the women had been trained to go immediately to the toilet and wash their hands. Police said the four North Korean suspects who had left the country put the VX liquid on the women's hands.

Police cannot confirm whether the two women may have been given antidotes before the attack. Atropine can be injected after exposure and is carried by medics in war zones where weapons of mass destruction are suspected.

Malaysia's finding that VX killed Mr Kim boosted speculation that North Korea orchestrated the attack. Experts say the oily poison was almost certainly produced in a sophisticated state weapons laboratory, and North Korea is widely believed to possess large quantities of chemical weapons including VX.

North Korea is trying to retrieve Mr Kim's body, but has not acknowledged that the victim is Kim Jong Un's half brother.