North Korean state media never mentioned Kim's children for years. But in November, at the launch of the country's most powerful ballistic missile, Kim was pictured with his daughter in tow.

There have since been several high-profile appearances by the daughter, who has never been named by Pyongyang. South Korea's spy agency believes she is Kim's second child, called Ju Ae.

The state-run Korea Stamp Corporation unveiled a new series of stamps on Tuesday commemorating the November 18 missile launch -- five of them featuring Kim and his daughter.

The corporation described the unnamed girl as Kim's "beloved daughter" in captions describing the stamps on its website. The series will be released on Friday.

Some experts say the daughter's appearances indicate that she is being groomed as the next leader, and that the stamps further cement her place in the cult of personality surrounding the ruling Kim family.

The stamps "look like the official start of Kim Ju Ae's life as her father's successor", An Chan-il, a defector-turned-researcher who runs the World Institute for North Korea Studies, told AFP.

But others said it was still too early to say for sure, given that Kim is still in his 30s and the daughter's name has never been mentioned officially.

"Throughout North Korean history, whenever it officially named the country's heir apparent, they made sure people knew the name of the person," Yang Moo-jin, a professor at the University of North Korean Studies in Seoul, told AFP.

"It's still very possible that Ju Ae is just being used for (propaganda)... while Kim's eldest son is being groomed as a successor behind closed doors."