Mr. Kim left for China on a private train on Monday afternoon accompanied by his wife, Ri Sol Ju, and senior North Korean officials, including Kim Yong Chol and Ri Yong Ho, North Korea's state-run KCNA news agency said.
Kim agreed to travel to Seoul for the first time as a North Korean leader "at an early date" during his summit meeting with South Korean President Moon Jae-in in September.
The reported meeting Monday came a day after President Trump told reporters a second location for the second U.S.
In this photo released on Tuesday, Jan. 1, 2019, North Korean leader Kim Jong Un delivers a New Year's speech in North Korea.
South Korean lawmaker Kim Min-ki said an official from Seoul's National Intelligence Service shared the information during a closed-door briefing.
A former North Korean diplomat who staged a high-profile defection to the South on Saturday urged an old colleague who has gone missing in Italy to defect to Seoul, following a report that he was seeking asylum in the United States.
China's official Xinhua news agency also confirmed that Mr. Kim is visiting from Monday to Thursday at Mr. Xi's invitation.
Despite Trump's repeated assurances that another summit will allow he and Kim to make a grand deal to settle the nuclear standoff and change a relationship marked by decades of animosity and mistrust, outside analysts are highly skeptical that the North will easily abandon a nuclear arsenal constructed in the face of deep poverty and likely seen by Kim as his only guarantee of regime survival.
Diplomatic sources said the last Italy heard of him was when officials received a note a year ago from the North Korean government saying that Mr Jo was being replaced.
Trump earlier said that the U.S. and North Korea are in talks over where to hold their second summit, adding that the decision on the venue will be announced in the "not-too-distant future".
Kim is expected to be in China from today until January 10. "The three summits with Xi Jinping in 2018 seem to have made Kim Jong Un very optimistic".
To defect to the South is an "obligation, not a choice" for North Korean diplomats committed to unification, Mr Thae said, calling Seoul "the outpost" for that task. "Oh, and we will take South Korea along", he added.
At the time, the two leaders signed a vaguely-worded pledge on denuclearisation of the Korean Peninsula, but progress has since stalled with Pyongyang and Washington arguing over their agreement's interpretation.