Secretive North Korea has never revealed her name or age, but South Korean intelligence officials believe she is the daughter identified as Ju Ae by former American basketball player Dennis Rodman. Rodman spent time with Kim's family in 2013 and described holding her as a baby.

She was seen just behind her father as they stepped off the armored train they used to travel overnight from Pyongyang to the Chinese capital Beijing, where Kim Jong Un attended a massive military parade on Wednesday staged by China to commemorate Japan's surrender ending World War Two in the Pacific.

"Right now, Ju Ae is the front runner as next supreme leader of North Korea," said Michael Madden, a North Korea leadership expert at the US-based Stimson Center. "She is getting practical protocol experience which should serve her well as North Korea's next leader or a core elite."

It is the first time she has accompanied Kim Jong Un outside North Korea – an experience that neither her father nor powerful aunt ever had, Madden said.

"She is getting valuable experience greeting and interacting with foreign leadership and other elites," he said.

Analysts said there is no evidence Kim Jong Un ever accompanied his father, Kim Jong Il, on overseas trips. Jong Il did make foreign trips in the 1950s with his father, North Korea's founder Kim Il Sung.

North Korea's tightly controlled state media had revealed nothing about Kim Jong Un's children until Ju Ae was first shown to the world accompanying her father to the launch of a massive intercontinental ballistic missile in 2022.

There is still little known about other Kim children.

South Korea's intelligence agency considers Ju Ae to be the most likely successor so far, despite questions over whether she can ultimately rise to the top of the male-dominated dynasty.

Estimated to be about 13 years old, Ju Ae has attended increasingly high-profile events, including her diplomatic debut at a Russian Embassy event in May.

"The scope of her public appearances certainly has expanded from military-related sites to political and economic events over the years," said Rachel Minyoung Lee, another researcher with the Stimson Center. "If this is part of a succession campaign, this would certainly help with that effort as it would be seen as Kim Ju Ae's debut on the international stage."

While it's premature to say conclusively whether this visit means she is Kim Jong Un's successor, it could still help broaden her horizons, said Lee, adding that how the North Korean media covers Ju Ae in China would give better insight.