Citing an unnamed "person knowledgeable about the matter", the paper said on Monday many details of Kim Jong Nam's relationship with the CIA remain unclear.

The US Central Intelligence Agency has declined to comment.

The Journal quoted the person as saying the 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.

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.

Fifield says Kim Jong Nam usually met his handlers in Singapore and Malaysia.

The book says 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 $US120,000 ($A172,000) in cash, which could have been payment for intelligence-related activities or earnings from his casino businesses.

South Korean and US officials have said 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 him 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.

Australian Associated Press