The National Intelligence Service (NIS) told lawmakers that its assessment was not based on indirect inferences but on what it described as "credible intelligence data" collected by the agency, according to briefings from members of the ruling and opposition parties after a closed parliamentary meeting.
The NIS stated that the purpose of showing the daughter driving a tank was to highlight her alleged military capabilities and dispel doubts about the female successor, lawmakers said.
North Korea's state media KCNA last month released photos of Kim and his daughter driving a new tank, following earlier photos showing her shooting a rifle at a firing range and using a pistol.
"Such scenes aim to pay 'tribute' to Kim's public military appearances in the early 2010s, when he was preparing to succeed his father," said Park Sun Won, a lawmaker from the ruling Democratic Party.
The repeated presence of Ju Ae at defense and military-related events aims to alleviate doubts about a female successor and accelerate the construction of a narrative of succession, lawmakers said, citing the NIS.
Lawmakers previously stated that the agency believes her increasingly prominent role suggests she is already being treated as the de facto second-highest figure in North Korea's leadership.
Lee Seong Kweun, a lawmaker from the People Power Party, said the NIS noted that suggestions that Kim's younger sister Kim Yo Jong might be dissatisfied with the focus on Ju Ae were incorrect, as Kim Yo Jong does not have independent power.