Kim Jong Nam, the half-brother of the North Korean leader, died a "very painful death" within 20 minutes of having poison smeared on his face, Malaysian authorities said.

This suggests he was exposed to a huge amount of VX, a nerve agent so lethal it is classified as a weapon of mass destruction, S. Subramaniam, Malaysia's Health Minister, said.

"The absorption level was so rapid that within a few minutes, the guy had symptoms," Subramaniam told reporters in Kuala Lumpur. "From the time of the onset, he died within 15 to 20 minutes."

Kim Jong Nam, the estranged older brother of North Korean leader Kim Jong Un, was attacked on February 13 in the busy Kuala Lumpur airport terminal by two women who wiped a substance on his face and in his eyes. They then fled while their victim sought help, but he died in an ambulance on the way to hospital.

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Malaysian police said that the autopsy showed Kim Jong Nam died from exposure to VX, a chemical weapon that is especially lethal when absorbed through the skin or mucous membranes.

All suspicions for the attack rest with the regime in Pyongyang. Malaysia has implicated eight North Koreans, including one diplomat said to be hiding in the North Korean Embassy in Kuala Lumpur, and a scientist whom it now has in custody.

South Korea has bluntly blamed the assassination on Kim Jong Un, accusing him of trying to eliminate potential rivals to his power, while the United States has decided not to issue visas for North Korean diplomats who were due to arrive in New York this week for talks with former American officials. The decision was made after Malaysia announced the finding of VX.

VX is banned under the international Chemical Weapons Convention, but North Korea is not a signatory. Pyongyang is thought to have the world's third-largest stocks of chemical weapons, behind the United States and Russia, and is believed to have been pursuing VX, according to the Nuclear Threat Initiative website.

VX is a nerve agent that stops muscles from being able to switch off, meaning that they work too hard and tire out, shutting down. This stops the major organs, including the lungs, from being able to work, leading to death by muscle paralysis.

"The muscle goes into a state of permanent contraction," Subramaniam said. The dose that the 45-year-old victim received was "so high" that his heart and lungs would have been affected quickly, he said.

After the autopsy results - and 11 days after the attack - teams in hazmat suits swept the airport terminal for traces of VX or other toxins but found none and declared the terminal safe.

The medical staff who helped Kim Jong Nam are also being monitored, but none has shown symptoms, Subramaniam said.

Questions are being raised about how the two women who carried out the attack, apparently without wearing gloves, managed to survive. Security camera footage shows them going to the bathroom immediately after the attack, presumably to wash their hands.

Siti Aishah, who is Indonesian and was reportedly the first to apply the oil-like substance to Kim Jong Nam, told police she vomited in a taxi after leaving the airport and continued to feel unwell. She is being tested.

Aishah has repeated to police that she was tricked into carrying out the attack, was told that it was a prank and was paid for taking part. But security footage shows the women acting with determination and immediately rushing off after the attack.

Malaysian authorities say that no one from Kim Jong Nam's family has come forward to claim his body. They have been asking for a DNA match to release the body, and there were reports last week that his 22-year-old son had arrived from Macau, but these turned out to be false.

North Korea has angrily denounced every part of the investigation, accusing the South Korean Government of persuading Malaysia to "besmirch" Pyongyang's reputation by laying the blame on it.

North Korea's Ambassador in Kuala Lumpur strongly objected to an autopsy being carried out at all, saying that the man - whom he did not identify - was carrying a diplomatic passport and was therefore not subject to Malaysian laws.

Malaysia has, however, insisted that it will follow all usual procedures for investigating a suspicious death.