Kim's father met then president Jiang Zemin in China in 2000 and a summit with the South was held later that year.
"The government is closely communicating with relevant countries and monitoring the situation", South Korea's presidential Blue House said in a statement via a messaging app earlier on Monday.
The arrival of a special train in Beijing and unusually heavy security at a guesthouse where prominent North Koreans have stayed in the past have raised speculation that Kim Jong Un is making his first visit to China as the North's leader.
Kim Jong Un is reportedly in China.
CBS News correspondent Ben Tracy reports that China's Foreign Ministry insisted Tuesday morning it had "no information" to share on whether Kim was in China, and China's censors have been furiously scrubbing speculation on social media about a visit - which, in China, tells you something. Since succeeding his father as leader in 2011, Kim has touted an image of his country as diplomatic equal to China and it's unlikely he would sneak into Beijing for his first face-to-face meetings with the Chinese leadership, the experts said.
Kyodo, citing sources close to the matter, said the visit of the official was meant to improve ties between Beijing and Pyongyang that have been frayed by North Korea's pursuit of nuclear weapons and China's backing of tough sanctions against North Korea at the United Nations Security Council.
North Korea's leader Kim Jong Un is believed to have wrapped up an unprecedented whirlwind trip to the Chinese capital, ahead of a planned summit with his South Korean counterpart and a potential meeting with U.S. President Donald Trump.
Ahead of his proposed summit with US leader Donald Trump, Bloomberg reported that Kim was in Beijing, citing three sources.
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The manager of a shop outside Beijing railway station described seeing "unusual" scenes on Monday afternoon.
According to the sources, the secretive visit is aimed at improving ties with Beijing.
A video that aired on NTV also showed a motorcade of black limousines waiting at the train station and rows of Chinese soldiers marching on what appeared to be a train platform.
The younger Kim had always been expected to make a visit to the capital of his country's most important ally and chief economic partner. Tensions were heightened in 2017 after a string of missile tests, which rankled China's leaders and resulted in multiple rounds of United Nations sanctions targeting North Korea's ability to bring in cash.
But Haenle says Xi may also have a vested interest in a Kim visit.
Kim Jong Il, who was known to be fearful of flying, visited China several times on his private armoured train. The envoy could potentially seek Chinese commitment for future support should North Korea's talks with rivals fall through, said Du Hyeogn Cha, a visiting scholar at Seoul's Asan Institute for Policy Studies.
Mending his relationship with China would be huge for Kim ahead of the summits with South Korea and the U.S. It would make North Korea less isolated, and China could use its influence to help Kim get what he wants out of the upcoming diplomacy.
"If North Korea speaks with the USA 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", he said. The office says it also can not confirm a report that the train carried Kim's sister.