Analysts say the visit may be part of a broader shift by the North to pivot from a confrontational stance to diplomatic outreach.
The most recent trip was in August 2011, months before Kim Jong Il's death, when he visited Moscow to meet with then President Dmitry Medvedev.
On Tuesday, a heavily guarded motorcade was seen in the centre of Beijing, while police blocked off streets around the main railway station.
According to South Korea's Chosun Ilbo newspaper, Mr Kim has not been seen in public since he met a South Korean delegation on March 5th amid reports he is gearing up for scheduled summits with South Korea and the US.
Critics of the president, though, said agreeing to the meeting signalled naivete, bestowing enormous prestige on Kim without any concessions having been made in advance by Pyongyang. China is by far North Korea's most significant link to the outside world, and it has been highly critical of unilateral US sanctions targeting the North that go beyond those approved late previous year by the U.N. Security Council.
The Blue House official said South Korea had been aware of "related movements" in North Korea, such as the train, for a few days but he could not confirm whether Kim or another high-ranking North Korean official was visiting China. North-Korea has remained a long-standing ally of China.
China has been largely sidelined by the developments between the two Koreas and the U.S., although it has repeatedly given public backing for talks between Washington and the Koreas.
Top Chinese, US Officials To Continue Trade Talks
Xi approaches the escalating trade spat from a position of strength at home, after securing an indefinite term as China's leader. The president is also asking Treasury Secretary Steven Mnuchin to come up with a list of restrictions on Chinese investment.
"The government is closely communicating with relevant countries and monitoring the situation", a South Korean government spokesman said on Monday. The meeting could be a new beginning in the strained ties between Beijing and North Korea. "If the meeting fails, the United States could declare that diplomacy has failed and shift to a more coercive approach or even a military strike".
Paul Haenle, a former White House official on China affairs and now the director of the Carnegie-Tsingua Center, says North Korea is extremely keen to get relief from the crippling global sanctions against it, and "China would be a key part of that effort".
According to Russian official Konstantin Pulikovsky, who accompanied Kim Jong Il and his entourage on a three-week journey to Moscow in 2001 and wrote about the experience in a 2002 memoir, "Orient Express: Through Russia with Kim Jong Il", meals on the train would often last for four hours, and included fresh live lobster or other delicacies.
North Korean leader Kim Jong Un at the military parade celebrating the 70th founding anniversary of the Korean People's Army in Pyongyang in February 2018.
There were also reports about heavy security on the China-North Korea border and later at a guesthouse where prominent North Koreans have stayed in the past. The approach led to plans for a summit between Moon and Kim.
There were posts on Chinese social media talking about the possibility Kim Jong Un was in China, some citing family members in Dandong.
What it comes down to, he said, is "neither North nor South Korea can afford to entirely ignore their giant allies, much as they may want to".