There is growing speculation that a senior North Korean official who has arrived unexpectedly in Beijing is Supreme Leader Kim Jong-un.
Japanese media outlets first reported that a high-profile figure had arrived via a North Korean diplomatic train and was met with tight security.
South Korea said it did not know the official’s identity, but that it was “carefully watching” the situation.
It would be Mr Kim’s first foreign visit since taking office in 2011.
There has been no official comment from China or North Korea, but such a visit would be seen as a significant development.
Last month, US President Donald Trump accepted an unprecedented invitation to meet Mr Kim, and officials are believed to be working behind the scenes to work out the complex diplomatic requirements.
Analysts have said the North Korean and Chinese leaders would likely meet before that summit goes ahead. China is North Korea’s main economic ally.
Footage of the train from the Tokyo-based Nippon News Network showed a green carriage with yellow horizontal lines.
The channel said it resembled one used by Mr Kim’s father and predecessor Kim Jong-il – who had a fear of flying – when he visited Beijing in 2011.
Kim Jong-il’s trips to China were only confirmed after his departure.
Source: BBC