The youngest son of North Korean leader Kim Jong-il is said to be on a visit to China in what may be a further clue to the reclusive state's succession plans.
Kim Jong-un was reported by South Korean media to have taken the train to China, the North's main ally.
It is believed to be the younger Kim's first such trip since he was given key military and political posts last year, the BBC reports.
Current leader Kim Jong-il is 69 and has had a stroke, prompting fears for the stability of the nuclear state.
"Kim Jong-un arrived in China early this morning via Tumen. But we don't know where he is now," South Korean news agency Yonhap quoted a source in the north-eastern Chinese border city as saying.
The BBC reports it is unclear exactly where Mr Kim may be heading: to Beijing, for key meetings with North Korea's political and economic guardians, or to development projects in the border region, to showcase him as a future leader.