Ri Jong Chol, who had been held since February 17, was released because of a lack of evidence to charge him and was handed over to immigration officials for deportation because he did not have valid travel documents, Deputy Prime Minister Ahmad Zahid Hamidi said.

Immigration director-general Mustafar Ali confirmed that Ri had flown out of the country, escorted by two North Korean Embassy officials, and that "he is blacklisted from entering Malaysia".

Police never said what they believed Ri's role was in the attack on the half-brother of North Korea's leader at Kuala Lumpur's airport.

Malaysia is looking for seven other North Korean suspects, four of whom are believed to have left the country on February 13, the day Kim died.

Two other people remain in custody, both women - one Indonesian, one Vietnamese - accused of smearing Kim's face with VX, a banned nerve agent considered a weapon of mass destruction.

Police have said three other North Korean suspects, 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 the country.

National police chief Khalid Abu Bakar said an arrest warrant was issued on Friday for the Air Koryo employee, Kim Uk Il.

Police say he arrived in Malaysia on January 29, about two weeks before Kim was killed.

Malaysian authorities have not said why they want to arrest Kim Uk Il.

Kim Yong Nam's death has unleashed a diplomatic battle between Malaysia and North Korea.

On Thursday, Malaysia announced it is scrapping visa-free entry for North Koreans.

Malaysia's foreign ministry ramped up the pressure on Friday, saying it was "greatly concerned" about the use of the nerve agent.

"Its use at a public place could have endangered the general public," the ministry said in a statement.

Malaysia has not directly accused North Korea of being behind the killing, but the statement came hours after a North Korean envoy rejected a Malaysian autopsy finding that VX killed Kim, saying the man probably died of a heart attack because he suffered from heart disease, diabetes and high blood pressure.

Ri Tong Il, a former North Korean deputy ambassador to the United Nations, also said that if VX had been used, others besides Kim would have been killed or sickened.

The women charged with murdering Kim at an airport terminal were caught on grainy surveillance video smearing what Malaysian authorities say was VX on his face and eyes, though both reportedly say they were duped into thinking they were playing a harmless prank.

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

Malaysia's finding that VX killed 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 Kim's body, but has not acknowledged that the victim is Kim Jong Un's half-brother, as Malaysian government officials have confirmed.

North Korea refers to the victim as Kim Chol, the name on the diplomatic passport he was carrying when he died.

Malaysian police are doing the same; they are seeking next-of-kin who could provide a DNA sample and make a positive identification, steps they say are needed for authorities to give up the body.

Kim is believed to have two sons and a daughter with two women living in Beijing and Macau.

Khalid Abu Bakar, the national police chief, brushed off the North Korean envoy's claim of a heart attack.

"We have our experts who are qualified to determine the cause of death of Kim Chol.

"Our investigations, supported by expert reports, confirmed that Kim Chol was murdered.

"North Korea can say what they like but the facts remain," he told the AP.

Malaysian police said the female suspects had been trained to go immediately to the bathroom and wash their hands after attacking Kim.

The police said the four North Korean suspects who left the country on the day of the killing put the VX liquid on the women's hands.

Police cannot confirm whether the two women may have been given antidotes before the attack.

An antidote, atropine, can be injected after exposure and is carried by medics in war zones where weapons of mass destruction are suspected.

North Korea has a long history of ordering killings of people it views as threats to its regime.

Kim Jong Nam was not known to be seeking political power, but his position as eldest son of the family that has ruled North Korea since it was founded could have made him appear to be a danger.

Kim Jong Nam reportedly fell out of favour with his father, the late Kim Jong Il, in 2001, when he was caught trying to enter Japan on a false passport to visit Tokyo Disneyland.