Photo/IllutrationThis combination of file photos shows Kim Jong Nam, left, in Narita in 2001, and North Korean leader Kim Jong Un in Pyongyang in 2016. (AP file photos)

SEOUL--South Korean agencies said Tuesday they could not confirm a report that the half brother of North Korean leader Kim Jong Un was a U.S. intelligence source and had traveled to Malaysia to meet his CIA contact before being assassinated there in 2017.

Officials at South Korea's National Intelligence Service and Unification Ministry, which oversees ties with North Korea, said they couldn't confirm Monday's report by The Wall Street Journal, which attributed the details to an unidentified "person knowledgeable about the matter."

It's extremely difficult for outside governments and media to verify information about North Korea and members of its secretive ruling family because Pyongyang closely watches visitors and enforces a stringent information blockade on its citizens. Outside media have a checkered history of relying on sources purporting to reveal secret information about the North that's later proven incorrect or incomplete.

While Kim Jong Nam spent much of his life abroad after falling out of favor with his family, he was constantly aware of the monitoring presence of Pyongyang and obsessed by fears that he would get assassinated, South Korean officials have said. Following his death, the NIS told South Korean lawmakers that North Korea had tried for several years to kill him and that he sent a letter to Kim Jong Un in 2012 begging for the lives of himself and his family.

The Journal said Kim Jong Nam met on several occasions with CIA operatives, but also that many details of his relationship with the agency remain unclear.

Messages left with the CIA weren't immediately returned.

Kim Jong Nam was killed on Feb. 13, 2017, when two young women smeared VX nerve agent on his face at a Kuala Lumpur International Airport terminal.

Murder charges were dropped against the women earlier this year. They had been accused of colluding with four North Koreans who prosecutors said had fled the country the day of the attack.

Lawyers for the women have said they were pawns in a political assassination with clear links to the North Korean Embassy in Kuala Lumpur. The United States and South Korea, too have blamed North Korea, but Malaysian officials never officially accused Pyongyang of involvement.

Kim Jong Nam was the eldest son in the current generation of North Korea's ruling family and could have been seen as a threat to Kim Jong Un's rule.