The half-brother of North Korean leader Kim Jong-un, who was killed by two women who smeared his face with the nerve agent in an airport, was reportedly a secret CIA informant.

The Wall Street Journal has spoken to a secret source who claims there was a "nexus" between the US spy agency and Kim Jong Nam — and that he had met with operatives several times.

They added that Kim — who lived mainly in the Chinese enclave of Macau —was killed while on a trip to meet his CIA contact person and that he was "almost certainly" in contact with security services of other countries.

The source told the Journal that the CIA, was relieved that his assistance didn't come out in the wake of that killing.

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Kim was killed at a Kuala Lumpur airport terminal on the morning of February 13, 2017.

After being ambushed by two women, who smothered a damp rag on his face, the dictator's half-brother approached security guards who took him to the airport's medical centre.

He was dead within 20 minutes of arriving and it was later found he had been killed with a deadly nerve agent.

Last month, Malaysian authorities released the two women who carried out the killing —Siti Aisyah, from Indonesia, and Doan Thi Huong, from Vietnam — believing they had been duped into carrying out the attack.

The pair said they thought they were participating in a prank for a TV show and did not know they actually were taking part in a high-profile murder.

Siti Aisyah, left, and Doan Thi Huong, right, were on trial for the killing. Photo / AP
Siti Aisyah, left, and Doan Thi Huong, right, were on trial for the killing. Photo / AP

Lawyers for the women have previously said they were pawns in a political assassination with clear links to the North Korean Embassy in Kuala Lumpur, and that the prosecution failed to show the women had any intention to kill. Intent to kill is crucial to a murder charge under Malaysian law.

Malaysian officials have never officially accused North Korea and have made it clear they don't want the trial politicised.

It's understood that Mr Kim had once been seen as the heir of the North Korean dictatorship.

He was sent abroad at a young age and returned to Pyongyang after his father became leader in 1994, with many assuming he was being groomed for the top job.

However, he reportedly fell out of favour in 2001 after he was caught trying to enter Japan on a false passport, saying he wanted to visit Tokyo Disneyland.

He had been living abroad for years and at the time of his death was travelling on a North Korean diplomatic passport under the name "Kim Chol".

He was educated in Moscow and Switzerland and didn't even meet his grandfather, North Korea's founding President Kim Il-Sung, until he was five years old.

His birth was considered shameful because his father and actress mother, Song Hye-rim were not married.

He was also known as a playboy because of his vice for gambling, drinking and women.

He told Japanese journalist Yomi Gomi in 2010 how he didn't know his sibling ruler, revealing: "I'm his half-brother, but I've never met him so I don't know."

He also predicted the fall of the regime and spoke of his disdain over its dynastic succession.

"I'm concerned how Jong-un, who merely resembles my grandfather, will be able to satisfy the needs of North Koreans," he told Gomi.

"Kim Jong-un is still just a nominal figure and the members of the power elite will be the ones in actual power. The dynastic succession is a joke to the outside world.

"The Kim Jong-un regime will not last long. Without reforms, North Korea will collapse, and when such changes take place, the regime will collapse."

North Korea has a long history of ordering killings of people it views as threats to its regime, though Kim was not thought to be seeking influence over his younger brother.

But he had spoken out publicly against his family's dynastic control of the North Korea.

North Korea has denied any role in the killing — suggesting he died of a heart attack and accused Malaysia of working with South Korean and other "hostile forces" in blaming Pyongyang.

The CIA has declined to comment on the Wall Street Journal reports and Chinese officials did not respond to the newspaper's request for comment.