North Korean leader Kim Jong Un has selected his daughter as his heir, South Korea's spy agency told lawmakers on Thursday.

Kim Ju Ae - who is believed to be 13 - has in recent months been pictured beside her father in high-profile events like a visit to Beijing in September, her first known trip abroad.

The National Intelligence Service (NIS) said it took a "range of circumstances" into account including her increasingly prominent public presence at official events" in making this assessment.

The NIS also said it would keep close tabs on whether she will attend the North's party congress later this month - its largest political event that is held once every five years.

The party Congress is where Pyongyang is expected to give more details about priorities like foreign policy, war planning and nuclear ambitions for the next five years.

On Thursday lawmaker Lee Seong-kwen told reporters that Ju Ae, who was previously described by the NIS as being "trained" to be a successor, was now at the stage of "successor designation".

"As Kim Ju Ae has shown her presence at various events, including the founding anniversary of the Korean People's Army and her visit to the Kumsusan Palace of the Sun, and signs have been detected of her voicing her opinion on certain state policies, the NIS believes she has now entered the stage of being designated as successor," Lee said.

Ju Ae is the only known child of Kim Jong Un and his wife, Ri Sol Ju. The NIS believes Kim Jong Un has an older son, but this son has never been acknowledged nor shown on North Korean media.

News of Ju Ae's existence first emerged through an unlikely source: the American basketball player Dennis Rodman, who revealed to The Guardian newspaper back in 2013 that he "held baby Ju Ae" during a trip to the secretive state.

Ju Ae made her first appearance on state television in 2022. She was shown inspecting North Korea's latest intercontinental ballistic missile while holding her father's hand.

She has since made frequent appearances on state media, softening her father's image of a ruthless dictator. She accompanied him to Beijing for China's largest-ever military parade, where she was seen stepping off his armoured train at Beijing Railway Station.

She is often seen wearing her hair long, which is forbidden for her peers, and wearing designer clothes, which are out of reach for most in her country.

Another lawmaker, Park Sun-won said the role Ju Ae had taken on during public events indicated that she has started to provide policy input and is being treated as the de facto second-highest leader.

The North Korean power had passed down the three generations of the Kim family, and it is widely believed that Kim Jong Un will pass on the throne to Ju Ae.

In recent months, she was shown standing taller than her father, walking beside him, rather than following him.

In North Korea, where photos published by the state media are believed to carry a great symbolic weight, it is rare for individuals other than Kim Jong Un to be positioned equally prominently in the frame.

Although the South Korean spy agency now believes Ju Ae is the designated heir, it still raises questions.

It is puzzling why Ju Ae, a daughter, would be selected as the heir above an older son in North Korea's deeply patriarchal society.

Many defectors and analysts had previously dismissed the idea of a woman leading North Korea as an unlikely scenario, referring to the country's entrenched traditional gender roles. But Kim Jong Un's sister - Kim Yo Jong - does offer a precedent for female authority in the regime.

Kim Yo Jong currently holds a senior position in the Central Committee of the Workers' Party of Korea, and is reported to have influence over her brother.

However, it is also a mystery why Kim Jong Un, who is still young and appears relatively healthy, is already designating a 13-year-old child as his heir now.

It is unclear what changes Ju Ae's succession may bring to North Korea.

Many North Koreans hoped that Kim Jong Un, a Western-educated young man, would open their country up to the outside when he succeeded his father.

Yet such hope was unfulfilled. Whatever plans this teenager may have for her country, she would likely have the singular power to shape it however she likes.