Media playback is unsupported on your device Media caption Rupert Wingfield-Hayes: Three reasons why the use of VX is so extraordinary

Kim Jong-nam, the half-brother of North Korea's leader, was killed by a highly toxic nerve agent, says Malaysia.

Mr Kim died last week after two women accosted him briefly in a check-in hall at a Kuala Lumpur airport.

Malaysian toxicology reports indicate he was attacked using VX nerve agent, which is classified as a weapon of mass destruction by the United Nations.

There is widespread suspicion that North Korea was responsible for the attack, which it fiercely denies.

It responded furiously to Malaysia's insistence on conducting a post-mortem examination and has accused Malaysia of having "sinister" purposes.

What does the toxicology report say?

Malaysia's police chief Khalid Abu Bakar said on Friday that the presence of the nerve agent had been detected in swabs taken from Mr Kim's eyes and face.

One of the women Mr Kim interacted with at the airport on 13 February had also fallen ill with vomiting afterwards, he added.

Mr Khalid said other exhibits were still under analysis and that police were investigating how the banned substance might have entered Malaysia.

"If the amount of the chemical brought in was small, it would be difficult for us to detect," he said.

What is the deadly VX nerve agent?

Image copyright Science Photo Library Image caption Molecular model of VX nerve agent shows atoms represented as spheres

The most potent of the known chemical warfare agents, it is a clear, amber-coloured, oily liquid which is tasteless and odourless

Works by penetrating the skin and disrupting the transmission of nerve impulses - a drop on the skin can kill in minutes. Lower doses can cause eye pain, blurred vision, drowsiness and vomiting

It can be disseminated in a spray or vapour when used as a chemical weapon, or used to contaminate water, food, and agricultural products

VX can be absorbed into the body by inhalation, ingestion, skin contact, or eye contact

Clothing can carry VX for about 30 minutes after contact with the vapour, which can expose other people

Banned by the 1993 Chemical Weapons Convention

Read more about VX

Who could be behind the attack?

How could it have killed Mr Kim?

Media playback is unsupported on your device Media caption CCTV footage appears to show the moment Kim Jong-nam is attacked

Bruce Bennett, a weapons expert at the research institute the Rand Corporation, told the BBC it would have taken only a tiny amount of the substance to kill Mr Kim.

He suggests a small quantity of VX - just a drop - was likely to have been put on cloths used by the attackers to touch his face. A separate spray may have been used as a diversion.

Mr Khalid has previously said the fact the woman who accosted Mr Kim immediately went to wash her hands showed she was "very aware" that she had been handling a toxin.

It would have begun affecting his nervous system immediately, causing first shaking and then death within minutes.

More from expert Bruce Bennett

Is Kuala Lumpur airport safe?

The authorities say they intend to decontaminate the airport and areas the suspects are known to have visited.

VX is a v-type nerve agent, which means the substance can remain lethal for a long period of time.

"It's as persistent as motor oil. It's going to stay there for a long time... which means anyone coming in contact with this could be intoxicated from it," forensic toxicologist John Trestrail told the Associated Press news agency.

No passengers, airport workers or medical staff who treated Mr Kim were reported to have become ill in the aftermath of the incident, the news agency adds.

Tens of thousands of passengers are believed to have passed through the airport since the attack more than 10 days ago.

Who was Kim Jong-nam?

Image copyright AP Image caption North Korea has not identified the man who died as Kim Jong-nam, only as a North Korean citizen

The well-travelled and multilingual oldest son of late North Korean leader Kim Jong-il, he was once considered a potential future leader. He has lived abroad for years and was bypassed in favour of his half-brother, Kim Jong-un.

He had been travelling on a passport under the name Kim Chol. North Korea has yet to confirm that the deceased was actually Kim Jong-nam.

For many years, it was believed Kim Jong-nam was being groomed to succeed his father as the next leader.

But that appears to have come to an end in 2001 when Kim was caught sneaking into Japan on a fake passport.

He later became one of the regime's most high-profile critics, openly questioning the Stalinist policies and dynastic succession his grandfather Kim Il-sung began crafting in 1948.

Kim Jong-nam: North Korea's critic in exile

How did he die?

A woman was seen in CCTV footage approaching Mr Kim and wiping something across his face. He sought medical help at the airport, saying someone had splashed or sprayed him with liquid.

He had a seizure and died on the way to hospital.

His body remains in the hospital's mortuary, amid a diplomatic dispute over who should claim it.

Main players in mysterious killing

Who did it?

Malaysia says it was clearly an attack by North Korean agents. Four people are in custody, including one North Korean and the two women he interacted with at the airport. Seven North Koreans are being sought, including a diplomat.

There are a number of North Korean organisations capable of directing such an attack, including the exclusive Guard Command.

The North hit back at Malaysia on Thursday, saying it was responsible for the death of one of its citizens.

In response, Malaysian Foreign Minister Anifah Aman warned North Korean envoy Kang Chol on Friday that he would be expelled unless he stopped "spewing lies" about the attack.

Who in North Korea could organise a VX murder?

North Korea's history of foreign assassinations

Does North Korea have VX?

North Korea is one of just six countries not to have signed the Chemical Weapons Convention (CWC) arms control treaty banning the production, stockpiling and use of chemical weapons.

According to the Nuclear Threat Initiative project, however, North Korea is thought to have the third largest stockpile of chemical weapons, after the US and Russia.

South Korea's defence ministry estimated in 2014 that the North has somewhere between 2,500 and 5,000 tonnes of nerve agents in stock, with VX identified as among them.