Four suspects in the killing of Kim Jong-nam, the estranged half-brother of North Korean ruler Kim Jong-un, are North Korean spies, South Korea's intelligence service told members of parliament in Seoul.

Kim Jong-nam died February 13 at Kuala Lumpur's airport in what Malaysian police say was a well-planned hit by a Vietnamese woman and an Indonesian woman who separately wiped a liquid onto Kim's face.

South Korean lawmakers cited the National Intelligence Service as telling them in a private briefing on Monday that four of the North Koreans identified as suspects by Malaysian police investigating the death are from the Ministry of State Security, the North's spy agency.

The NIS was quoted as saying that two other suspects are affiliated with Pyongyang's Foreign Ministry, according to Lee Cheol-woo, one of the politicians who attended the briefing.

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Another legislator, Kim Byeong-ki cited the NIS as saying Kim Jong-un directed a "state-organised terror" to kill his brother.

The politicians did not say how the NIS got the information and if it elaborated on what specific roles these North Korean suspects performed.

The NIS has a mixed record on reporting developments in the secretive North. The agency said it cannot confirm its reported assessment on Kim Jong-nam's death.

Malaysia has not directly accused North Korea of having masterminded the Kim Jong-nam killing but is pursuing several North Korean suspects, including a diplomat at the North Korean Embassy in Kuala Lumpur.

Police last week identified the substance as the banned chemical weapon VX nerve agent, and Malaysia's health minister said on Sunday the dose was so high it caused "very serious paralysis" and killed him within 20 minutes.

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Malaysian officials have said four North Korean men provided the two women with the VX agent, then fled Malaysia the same day.

It was unclear if those four were the four North Korean spies cited by South Korea's intelligence agency.

North Korea has repeatedly criticised Malaysia's investigation and has not acknowledged the victim's identity.