Media playback is unsupported on your device Media caption Kim Jong-nam was poisoned with an extraordinarily potent chemical weapon called VX

South Korea's intelligence agency believes that four of the North Korean men suspected of involvement in the death of Kim Jong-nam are spies.

The older half-brother of North Korea's leader was poisoned at Kuala Lumpur airport, Malaysian police say.

Four of the seven suspects named by Malaysia work for Pyongyang's ministry of state security, intelligence officials in Seoul told MPs.

It is unclear which of the North Koreans wanted by Malaysia were meant.

Mr Kim died two weeks ago after two women accosted him in a check-in hall at the airport.

They say they thought they were taking part in a TV prank.

Image copyright AFP Image caption Two of the suspects wanted by Malaysian police were named as Hong Song Hac, 34, (top) and Ri Ji Hyon, 33

Mr Kim was given a very high amount of the toxic nerve agent VX and died in pain within 15-20 minutes, Malaysia's health minister said on Sunday.

Who in North Korea could organise a VX murder?

What is South Korea's take on the killing?

Unravelling the mystery of Kim Jong-nam's death

Malaysian police arrested a North Korean man named Ri Jong Chol a few days after the killing.

Six other North Korean men have been named as suspects or are wanted in connection with the death.

Four of them flew out of Kuala Lumpur airport shortly after the attack on Mr Kim, returning to Pyongyang via a complicated route that avoided China.

The two other North Koreans named by Malaysian authorities are a senior official at the North Korean embassy in Kuala Lumpur and an employee of the state airline, Air Koryo.

While South Korea said on 19 February that it believed the North Korean regime was behind the death of Mr Kim, Malaysian authorities have not accused Pyongyang of carrying out the killing.

North Korea has said that Malaysia is responsible for the death of one of its citizens and is attempting to politicise the return of his body.

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 was Kim Jong-nam?

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.

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

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 authoritarian policies and dynastic succession his grandfather Kim Il-sung began crafting in 1948.

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