"Speculation is rife that Kim Jong-un may have arrived in Beijing by train", said Stephen McDonnell, the BBC's China Correspondent, in a tweet.
If Mr Kim was on board, it would be the first time he had left North Korea since becoming leader in late 2011 (as far as we know, at least).
Kim is expected to have a summit meeting with South Korean President Moon Jae-in in late April and with President Trump in May. Footage of the arriving train from North Korea appeared similar to the train Kim's father Kim Jong-il used for a trip to China shortly before he died.
Bolton, who served as a Fox News commentator, said the sooner Trump meets with North Korean leader Kim Jong-un, the better it would be.
Two sources in northeastern China also told Reuters that a North Korean visitor had crossed into Dandong by train.
The state-run Korean Central News Agency issued commentary last week that finally hinted at confirmation for the upcoming meeting between North Korea, the USA and South Korea. North Korea's closest ally recently placed a cap on oil exports to North Korea.
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On Monday, an armored train made its way through northeast China, disrupting the railway schedule and sparking social-media speculation that high-ranking North Korean officials were making their way to Beijing.
It is unknown what Kim would be doing in Beijing, with meeting coming just weeks after he announced plans to meet Trump.
Kim has not met with any major world leaders since assuming leadership of North Korea in 2011.
He said he had reservations about whether North Korea would actually get rid of all its nuclear weapons.The positions of John Bolton, Trump's pick to replace HR McMaster as national security adviser, were some cause for concern, Cohen added.
Residents of the border town of Dandong reported elevated security around the train station and local rumors that Kim Jong-un was passing through.
The Blue House in Seoul acknowledged the reports on the visit and confirmed that officials were monitoring the situation.