Kim Jong-nam, the half-brother of North Korean leader Kim Jong-un who was killed in Malaysia in 2017, had been an informant for the US Central Intelligence Agency, the Wall Street Journal reported on Monday (US local time).
The Journal cited an unnamed "person knowledgeable about the matter" for the report, and said many details of Kim Jong-nam's relationship with the CIA remained unclear.
Reuters could not independently confirm the story. The CIA declined to comment.
The Journal quoted the person as saying that "there was a nexus" between the CIA and Kim Jong-nam.
"Several former US officials said the half brother, who had lived outside of North Korea for many years and had no known power base in Pyongyang, was unlikely to be able to provide details of the secretive country's inner workings," the Journal said.
The former officials also said Kim Jong-nam had been almost certainly in contact with security services of other countries, particularly China's, the Journal said.
Kim Jong-nam's role as a CIA informant is mentioned in a new book about Kim Jong-un, The Great Successor, by Washington Post reporter Anna Fifield that is due to be published on Tuesday.
Ms Fifield said Kim Jong-nam usually met his handlers in Singapore and Malaysia, citing a source with knowledge of the intelligence.
The book says that security camera footage from Kim Jong-nam's last trip to Malaysia showed him in a hotel elevator with an Asian-looking man who was reported to be a US intelligence agent.
It said Kim's backpack contained $US120,000 ($NZ181,900) in cash, which could have been payment for intelligence-related activities, or earnings from his casino businesses.
South Korean and US officials have said the North Korean authorities had ordered the assassination of Kim Jong-nam, who had been critical of his family's dynastic rule. Pyongyang has denied the allegation.
Two women were charged with poisoning Kim Jong-nam by smearing his face with liquid VX, a banned chemical weapon, at Kuala Lumpur airport in February 2017.
Malaysia released Doan Thi Huong, who is Vietnamese, in May, and Indonesian Siti Aisyah in March.
According to the Journal, the person said Kim Jong-nam had travelled to Malaysia in February 2017 to meet his CIA contact, although that may not have been the sole purpose of the trip.
US President Donald Trump and Kim Jong-un have met twice, in Hanoi in February and Singapore last year June, seeming to build personal goodwill but failing to agree on a deal to lift US sanctions in exchange for North Korea abandoning its nuclear and missile programs.