The assassinated half-brother of North Korean leader Kim Jong-un was particularly close to their once-powerful uncle Jang Song-thaek, according to a new book on North Korea.

Anna Fifield, the Washington Post's Beijing bureau chief, said at a promotional event for her new book "The Great Successor" in Washington on Wednesday that Kim Jong-nam maintained close ties to those at the highest echelons of the North Korean state throughout his time in quasi-exile. 

She added that he remained particularly close to his uncle, Jang Song-thaek, the once-powerful political mentor of Kim Jong-un who was executed in 2013. 

Fifield said that Kim was an informant for the U.S.' Central Intelligence Agency, handing over information on his half-brother and the North Korean regime while contacting agents in Southeast Asian countries including Malaysia and Singapore. 

She added that Kim was a good source of information as he maintained contacts with high-ranking officials in his family’s regime.

Fifield noted that although Kim Jong-nam had lived outside North Korea for nearly 20 years and showed no interest in attaining personal power, he may have nonetheless been considered a political rival by Kim Jong-un due to the hereditary characteristics of the regime.

Kim Jong-nam, the eldest son of former North Korean leader Kim Jong-il, was assassinated by the nerve agent VX in a Malaysian airport in 2017, a move suspected to have been orchestrated by Pyongyang.