The son and heir apparent to Kim Jong-il, North Korea's leader, joined a delegation of senior military officials for a top-secret week-long visit to China in mid-June despite Beijing's claims that no such trip occurred.

The visit was intended to shore up support for the inexperienced Kim Jong-woon, Mr Kim's 26-year-old son, and reassure North Korea's closest ally that a smooth leadership transition is already under way, military, intelligence and diplomatic sources have told the Financial Times.