South Korea's spy agency said Thursday that North Korean leader Kim Jong Un's daughter, Ju Ae, who recently accompanied him on a high-profile visit to Beijing, is understood to be his recognised successor.

The teenager drew global attention earlier this month when she made her first official overseas trip with her father, as he met with China's Xi Jinping and Russia's Vladimir Putin.

Analysts have long seen her as Kim's likely successor, although some have suggested Ju Ae has an older brother who is being secretly groomed as the next leader.

South Korea's spy agency "assesses that she (Ju Ae) secured sufficient 'revolutionary narrative' needed to strengthen her position as a likely successor," following her China trip, lawmaker Lee Seong-kweun told reporters after being briefed by the agency.

"The agency perceives Kim Ju Ae as the recognised heir and sees her participation in the China visit as part of completing that succession narrative."

Seoul's spy agency said Ju Ae's status was evident in state media photos and a special documentary showing her accompanying Kim Jong Un in China.

North Korea's state media released a photo of her inside the country's bulletproof train alongside her father and other officials as they returned from Beijing.

Seoul's spy agency said it understood that Kim and his daughter "stayed at the North Korean embassy" while in China, where items and even waste were "transported by special aircraft".

"Strict protection of their leader's biometric information has long been part of North Korea's security protocol," Ahn Chan-il, a defector-turned-researcher who runs the World Institute for North Korea Studies, told AFP.

"The fact that Ju Ae received the same level of protection as Kim Jong Un once again demonstrates that she is a highly likely successor."

- 'Great person of guidance' -

Ju Ae was publicly introduced to the world in 2022, when she accompanied her father to an intercontinental ballistic missile launch.

North Korean state media have since referred to her as "the beloved child," and a "great person of guidance" -- "hyangdo" in Korean -- a term typically reserved for top leaders and their successors.

Before 2022, the only confirmation of her existence had come from former NBA star Dennis Rodman, who made a visit to the North in 2013.

Seoul had initially indicated that Kim and his wife Ri had their first child, a boy, in 2010, and that Ju Ae was their second child.

But in 2023, Seoul's unification minister said that the government was "unable to confirm" the existence of Kim's son.

MP Lee said rumours that Kim has another child, including one with a disability or studying abroad, are "not considered credible".

"In particular, in the case of studying abroad, the NIS noted that no matter how much one tries to conceal such a fact, it would inevitably become known, and thus the possibility is considered very low," he added.

Also on Thursday, South Korean President Lee Jae Myung said the previous Yoon Suk Yeol government sent drones over Pyongyang, now the subject of a special investigation.

Last year, North Korea said it "proved" South Korea's military sent a drone over Pyongyang to drop propaganda leaflets -- something South Korea's defence ministry has not confirmed.

South Korea has opened a special investigation into whether sending the drones was an illegal attempt to provoke the North and use it as a pretext to declare martial law.

Yoon was eventually ousted over his botched attempt to subvert civilian rule.