North Korean leader Kim Jong-il turns 67 on Monday with no clear successor to the world's first communist dynasty but with several potential candidates including a son once detained trying to sneak into Tokyo Disneyland.
Succession is one of the most closely guarded secrets in the highly secretive North but intelligence officials said the three men most likely to replace Kim were his eldest and youngest sons and a brother-in-law.
The issue of a successor became more urgent after Kim was suspected of suffering a stroke in August.
North Korea's founder Kim Il-sung tipped Kim Jong-il as his successor in 1974, giving his son decades to build trust with the country's powerful military before the elder Kim died in 1994.
In contrast, Kim Jong-il has given no indication which, or even if, one of his three known sons will take over the dynasty.
Last month the South's Yonhap news agency said he had picked his youngest known son, Kim Jong-un, to succeed him.
Senior officials of the powerful party apparatus, where Kim himself began his training as anointed leader before succeeding his father, were instructed to pass the message down the ranks, Yonhap quoted an intelligence source as saying.
There has been no confirmation of the report and intelligence sources told Reuters no candidate had emerged as the favourite. Some analysts also expressed scepticism about the report.
"There is no evidence in the propaganda to suggest any change yet," said Brian Myers, an expert on the North's state ideology at the South's Dongseo University.
"If they are really preparing or grooming somebody, it would have to be at a quicker pace ... and more explicit."
The Swiss-educated Jong-un is believed to have been born in late 1983 or early 1984 and has been described as his father's favourite and an intelligent and thoughtful man.
But his youth has been cited as a barrier to his rise to power in a society where the tradition of seniority is strong.
Embarrassed eldest son
The eldest son Jong-nam once fell out of favour when he was deported from Japan in 2001 on suspicion of trying to enter the country using a forged Dominican Republic passport in a bid to see Tokyo Disneyland.
The easy-going and portly Jong-nam has spoken to reporters several times over the last year, answering questions in Korean and near-perfect English.
"Jong-nam lives about half the year abroad and his father uses him for certain overseas tasks," one intelligence source told Reuters on condition of anonymity.
Middle son Kim Jong-chol, believed to be 27, is rarely mentioned as a potential successor.
Experts say North Korea could move into a collective leadership when Kim Jong-il dies with one of his sons named as figurehead leader but real power held by a group of officials from the ruling Workers' Party and the military.
Kim's inner circle is dominated by cadres in their 70s and 80s who could oversee a caretaker government.
Jang Song-taek, 63, husband of Kim's sister and a senior communist party official, is largely expected to play a major role in the leadership that survives Kim Jong-il, possibly as a caretaker or as the outright leader.
Jang has been mentioned often in North Korea's official media as joining Kim on his field visits over the past months while one of his allies was named the North's new defence minister.
"If he had a son in mind, regardless of which one, there would be no reason why he would not start now grooming this person," said Myers.
"It might just be the logical thing to think that Kim will leave this off to Jang Song-taek and leave his kids with a boatload of money with which they can buy themselves clean in the post unification Korea," Myers said.