SEOUL (AFP) - North Korea has up to 5,000 tonnes of chemical weapons, South Korean experts said Friday (Feb 24), including the toxin used to assassinate its leader's half-brother.

Traces of VX - a nerve agent listed as a weapon of mass destruction by the United Nations - were detected on swabs from the face and eyes of Kim Jong Nam, who was poisoned at Kuala Lumpur's international airport last week, Malaysian police said Friday.

Malaysian detectives are holding three people - women from Indonesia and Vietnam, and a North Korean man - but want to speak to seven others, four of whom are believed to have fled to Pyongyang.

South Korea's defence ministry said in its 2014 Defence White Paper that the North began producing chemical weapons in the 1980s and estimated that it has about 2,500 to 5,000 tonnes in stock.

North Korea has chemical weapons production facilities in eight locations including the northeastern port of Chongjin and the northwestern city of Sinuiju, it said in the 2012 edition of the document.

"North Korea is believed to have a large stockpile of VX, which can easily be manufactured at low cost," defence analyst Lee Il Woo at the private Korea Defence Network told AFP.

Developed some 100 years ago, VX can be produced at small laboratories or facilities producing pesticides, he said.

"Chemical and biological weapons can be delivered through various means such as artillery, missiles and planes", he added.

If absorbed through the skin, eyes or nose, just a tiny drop of the colourless, odourless nerve agent is enough to fatally damage a victim's central nervous system.

Military science professor Kim Jong-Ha at Hannam University said the North has 16 kinds of nerve agents including VX and sarin, used by a Japanese doomsday cult, Aum Shinrikyo, in the 1995 attack on the Tokyo subway system that killed 12 people.

It also possesses other lethal chemicals, including suffocating, blistering and blood agents, Kim said, as well as 13 types of biological weapons such as anthrax and bubonic plague.

Defence analyst Joseph S. Bermudez Jr. has said that North Korea "produces and possesses the capability to effectively employ throughout the Korean peninsula, significant quantities and varieties of chemical weapons", and could have as many as 150 chemical weapons warheads for ballistic missiles.

"It also has, to a lesser extent, the ability to employ these weapons worldwide using unconventional methods of delivery," he wrote on the closely-watched US-Korea Institute's website 38North in 2013.

There was a "growing body of evidence" that the North had an "ominous" history of proliferating chemical weapons capabilities to countries such as Syria and Iran, he added.

North Korea has not signed a global chemical weapons convention that prohibits the production, stockpiling and use of chemical weapons.

More than 160 countries signed the treaty, that went into force in 1997.

In a 2015 assessment, the Washington-based Nuclear Threat Initiative wrote: "North Korea claims that it does not possess chemical weapons.

"While assessing stockpiles and capabilities are difficult, the DPRK is thought to be among the world's largest possessors of chemical weapons, ranking third after the United States and Russia."