SEOUL - North Korea's ruling communist party will pledge loyalty Thursday to the youngest son of leader Kim Jong-Il, a report said, ahead of the birthday of a man widely-tipped to succeed his father.

The show of support for Kim Jong-Un, who turns 26 on Friday, will take place in the capital Pyongyang, said Opennews for North Korea, a Seoul-based media organisation run by North Korea watchers.

Party members have held a series of rallies in other cities this week to pledge loyalty to Jong-Un, NK Intellectuals Solidarity, a group of defectors, said in a report.

The South's unification ministry, which deals with cross-border relations, said it could not confirm the reports.

The regime has used Kim Jong-Il's birthday, a major national holiday, to stress loyalty to him.

Succession speculation has intensified since Kim senior, 67, suffered a stroke in August 2008. Widespread reports say he has chosen Jong-Un to inherit power.

There have been unconfirmed reports that Jong-Un was already given a post in the communist party in preparation for his eventual takeover.

But some analysts say Pyongyang is waiting for the right opportunity to announce Jung-Un's nomination as eventual successor.

Kim Jong-Il, who took over after his father died in 1994 in a long-planned move, has three sons from two marriages, according to outside analysts.

Information is scant about Jong-Un, the second son of Kim Jong-Il's third wife Ko Yong-Hee. No adult photo of him is publicly available.

Some reports say Jong-Un, born in 1984, attended an international school in the Swiss city of Berne under a pseudonym.

Kenji Fujimoto, a former personal chef to Kim Jong-Il, has described the third son as "a chip off the old block" who closely resembles his father physically and in terms of personality.