By Agence France-Presse
If confirmed, it would mark Kim's first trip abroad since coming to power in 2011 and signal an intriguing twist in a fast-developing diplomatic exercise that has opened the door to separate summits between Kim and the presidents of South Korea and the United States.
Some analysts had suggested China -- the North's only major ally -- had been sidelined by the overtures from Pyongyang to Seoul and Washington, but a visit by Kim would put Beijing firmly back at the centre of the diplomatic equation.
Bill Bishop, publisher of the Sinocism China Newsletter, said President Xi Jinping likely wanted to meet with Kim before a summit with US President Donald Trump, which could take place in May.
"They're concerned about being left out, with the North Koreans directly cutting a deal with the Americans that doesn't necessarily reflect Chinese interests," Bishop told AFP.
A possible visit was first reported by Japan's Kyodo news agency, citing unidentified sources as saying that a high-ranking North Korean official had arrived in the Chinese capital on Monday.
Japanese broadcaster Nippon TV showed footage of a train -- similar to that used for foreign visits by Kim's late father Kim Jong Il -- pulling in to Beijing Station and being met by a military honour guard and a convoy of black limousines.
At the Diaoyutai guest house, where Kim Jong Il stayed during his visits to Beijing, there was an unusually heavy police presence with officers stationed every 50-100 metres in front of the imposing compound.
An AFP photographer saw a motorcade of limousines leave the guest house under a police escort on Tuesday morning.
There was also heightened security at two possible venues for a high-level meeting -- the Great Hall of the People and Zhongnanhai, the central leadership compound next to Beijing's Forbidden City.
- Official silence -
There was no mention of any visit by either the Chinese or North Korean state media, and a Chinese foreign ministry official told AFP on Tuesday that it was "not aware of the situation".
China's Weibo micro-blog was censoring searches for Kim Jong Un's name and variations on it Tuesday. Beijing often tightens news controls during sensitive political periods.
South Korean broadcaster SBS TV said that guests at a hotel in the border city of Dandong, overlooking the train link from China to North Korea, had been asked to leave and curtains were drawn across the hotel windows.
The hotel will resume bookings on Wednesday afternoon, the broadcaster said, suggesting the train, and whoever might be on board, would have returned by then.
Kim Jong Il, who was known to be fearful of flying, visited China several times on his private, armoured train. His visits were confirmed by Chinese and North Korean state media only after he had left the country.
The younger Kim has not undertaken any official trip abroad since taking power following his father's death in 2011. And he has yet to host a single head of state, having snubbed the president of Mongolia who visited Pyongyang in 2013.
In Washington, the White House said it was unable to confirm Kim's presence in Pyongyang and the government in Seoul said only that it was closely monitoring the situation.
For decades Beijing has been Pyongyang's key diplomatic protector and main source of trade and aid, but their relationship has soured in recent years.
Kim broke with tradition by not travelling to Beijing to pay his respects to Xi after the Chinese leader came to power, and Beijing has become increasingly frustrated with its neighbour's nuclear weapon programme -- showing a new willingness to agree to, and enforce, tougher UN sanctions.
At the same time, Beijing fears the collapse of the regime in Pyongyang and the instability it would bring, potentially sending waves of refugees into China and the possibility of US troops stationed on its border in a unified Korea.
High-level inter-Korean talks are scheduled for Thursday to pave the way for a summit between Kim and South Korean President Moon Jae-in in late April. Discussions have also begun on a possible summit with Trump in May.