"It is unknown whether the North Korean train is moving to Beijing, but [Seoul] follows its movement, as there is a possibility that [North Korean] chairman Kim [Jong-un] is carrying out his fourth visit to China," the source told the Hankyoreh newspaper.
According to North Korean and Chinese media reports, the train, presumably carrying Kim, crossed into the Chinese border city of Dandong at around 10:15 p.m. local time (13:15 GMT). Shortly before its arrival, Chinese security officers have shut off entry points to the station. After the train moved further, having crossed the border, the cordon was lifted.
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The train is expected to arrive in Beijing by early morning Tuesday, media reports suggest.
Neither Chinese nor South or North Korean authorities have provided any comment so far.