Denuclearisation talks between the North and the U.S. have stalled, but speculation has mounted of a potential second summit between Kim - whose birthday is reportedly on Tuesday, although that has never been confirmed by Pyongyang - and Trump.
The source says the identity of the VIP traveler has yet to be confirmed, . adding the outer appearance of the special train used by North Korean leader Kim Jong-un is no different from other trains.
Reports of the North Korean leader's possible trip to China come after USA and North Korean officials are believed to have met in Vietnam to discuss the location of a second summit between Kim and President Donald Trump.
Officials from the Blue House in Seoul could not immediately be reached for comment by AFP.
According to the Korean Central News Agency, Kim was spotted traveling with his wife and various high-ranking officials including Kim Yong Chol, who is the vice chairman of the Central Committee of the North Korean ruling Workers' Party.
Rumors about Kim's visit to China began late Monday local time when a North Korean train painted green carrying a "senior North Korean official" was described crossing over the North Korea-China border, South Korea's Yonhap News Agency reported.
North Korea rarely confirms visits until they have been completed.
Last week, Trump received another letter from Kim.
At the first summit between the longtime adversaries in June, Trump and Kim agreed to work toward the Korean peninsula's denuclearization but with little apparent agreement on what that means.
"We are negotiating a location", he told reporters. "We're doing very well".
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High-level talks between the two countries were abruptly canceled in November a year ago at the request of North Korea, while progress on denuclearization talks has stalled since the two leaders' first historic meeting in Singapore in June.
Culminating in late 2017, Pyongyang carried out six atomic blasts and launched rockets capable of reaching the entire United States mainland, but has now carried out no such tests for more than a year.