There is widespread speculation that a senior North Korean official who made a surprise day-long visit to Beijing was 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 trying to verify the situation.
It would be Mr Kim's first foreign visit since taking office in 2011.
There has been no official comment from North Korea, while China's foreign ministry said it had "no information for the moment", but more would "be published in due course".
A visit by Mr Kim to Beijing 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 be likely to meet before that summit goes ahead. China is North Korea's main economic ally.
Japanese media reported on the train's arrival into Beijing on Monday evening. Footage of the train from the Tokyo-based Nippon News Network showed a green carriage with yellow horizontal lines.
People in Beijing described seeing "unusual" scenes, including tightened security outside the train station, and tourists being ushered out of the capital's Tiananmen Square, which usually signals a high-level meeting in the Great Hall of the People there, reports BBC.