Speculation is growing that North Korean leader Kim Jong Un has visited Beijing after a diplomatic train arrived from Pyongyang.
The train was met with tight security when it rolled into the Chinese capital, and a convoy of 20 cars was seen leaving the city's Diaoyutai State Guest House, where senior foreign leaders often stay during visits.
Police blocked roads near the building as the convoy travelled by.
Although there has been no official confirmation of the visit from China or North Korea, a presidential official in South Korea said Seoul was "watching things in Beijing very closely".
"Right now, the situation surrounding the Korean peninsula is moving very quickly and it would be inadvisable to think with prejudice," they added.
Heavy security was reported at the Friendship Bridge over the Yalu River which marks the border between China and North Korea before a train passed. The train has since left Beijing's station.
A Chinese government spokesman is expected to provide information on the reports "in due course".
Yoo Ho-yeol, a professor of North Korean studies at Korea University in Seoul, suggested the meeting was likely a bid by Mr Kim to "reaffirm close ties" with Beijing before a planned summit with US President Donald Trump.
He said: "If North Korea speaks with the US on its own it might feel it is at a disadvantage, but if it has China as an ally, Pyongyang may think it will be able to protect its interests and profits during the summits."
If the North Korean leader has been in Beijing, which diplomatic sources have said is "likely", it would mark his first foreign visit since taking office in 2011.
His father, Kim Jong Il, last visited Beijing in 2011 and did so aboard a train similar to the one spotted in the city on Monday.
It comes ahead of planned summits between North Korea, South Korea, and the US, with the three having agreed to talk to allay tensions over Mr Kim's nuclear ambitions.
His sister, Kim Yo Jong, who some analysts have suggested could be aboard the train, visited the South for the Winter Olympics in February and earlier this month senior officials from Seoul travelled to Pyongyang.
Mr Trump has since agreed to meet Mr Kim by May, with the latter's foreign minister's recent visit to Sweden fueling suggestions that the historic summit could take place in the country.
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