"Japanese media reported on Monday that a high-ranking Pyongyang official appeared to have arrived by train in Beijing".
If the reports are true, it would be Kim's first known foray outside North Korea since he took power upon his father's death in 2011.
There was high security in parts of Beijing where North Korean officials are believed to be visiting.
A spokeswoman for the Chinese Foreign Ministry said she was not aware of the situation and had no further comment. On Tuesday afternoon, a vehicle convoy led by a motorcycle escort headed in the direction of Beijing's main railway station.
Chung Seong-chang, a senior fellow at the Sejong Institute specializing in North Korea affairs, agreed that it was Kim Jong-un who made the trip, not his younger sister.
The sightings prompted a flurry of speculation that the North Korean leader could be in the Chinese capital ahead of his planned summit with South Korean President Moon Jae-in and a potential meeting with US President Donald Trump.
Kim Il-sung, Kim Jong-un's grandfather and the country's first leader after its establishment in 1948, arrived in China by train for his first visit to the country in 1982. "Either way, giving the impression of unity is good for both North Korea and China at the current moment", said Christopher Green, an expert on North Korea who is an adviser for the International Crisis Group.
He added that the White House was "looking forward to a potential summit some months in advance".
Heavy security was reported at the Friendship Bridge before the train passed from North Korea to China, and there were reports of it passing through several stations on the way from North Korea to Beijing.
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China shares a border with North Korea and is the isolated totalitarian state's traditional ally. "North Korea could reach out to China and Russian Federation in addition to South Korea and the United States from the perspective of foreign relations", one of them said.
Ties have been strained between Pyongyang and Beijing in recent times after the secretive state conducted a number of missile tests. "If the meeting fails, the U.S. could declare that diplomacy has failed and shift to a more coercive approach or even a military strike".
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Whoever was on the train, their visit to Beijing appeared to be short.
A Russian official who accompanied Kim Jong Il on a trip to Moscow in 2001 said the train was packed with cases of Bordeaux and Beaujolais from Paris.
Some analysts had suggested China - the North's only major ally - had been sidelined by Pyongyang's approaches to Seoul and Washington, but a visit by Kim would put Beijing firmly back at the centre of the diplomatic scrum. He noted that Xi probably had advice for Kim for the upcoming summit between Kim and Trump, which will be the first-ever such talk between the two countries' leaders.
North Korean news media has a history of publishing its former leader's China visits but only after he almost finished his trips, probably because of security reasons.
Reports from the U.S. quoted defence secretary Jim Mattis replying to a question about Kim's rumoured Beijing visit: "Kind of looked like he did, but I don't know".