A train reportedly carrying Kim left Beijing in the afternoon and is possibly headed back to North Korea, according to the Kyodo news agency.
For his part, Kim said North Korea will make efforts to "achieve results that will be welcomed by the global community" at the next summit with the United States, China's official news agency added.
The report came as Kim returned to Pyongyang after a trip to China, where he held talks with Xi and visited an economic development area in Beijing.
"China supports the DPRK's continued adherence to the direction of denuclearisation on the peninsula, supports the continuous improvement of inter-Korean relations", Xi was quoted as saying, using the initial of North Korea's official name, the Democratic People's Republic of Korea.
Kim Jong-un visits a Tong Ren Tang pharmaceutical plant in Yizhuang, Beijing on Wednesday.
The trip - Kim's fourth to China since March - suggests negotiations over North Korea's nuclear arsenal are gaining momentum after months without high-level diplomatic exchanges.
China is North Korea's only major ally, and Kim is likely consulting and co-ordinating with Chinese leader Xi Jinping and others ahead of a possible second summit with President Donald Trump. But talks have since stalled over disagreements about how to accomplish that. Jim Jatras also claimed that what President Trump and what his administration want are not "necessarily the same thing".
"Kim needs the support of Xi so as to ask the United States to make substantial steps, such as providing assistance to North Korea and normalising relations with North Korea", he said.
While Mr. Trump says he considers Xi key to enticing Kim into taking concrete steps toward denuclearization, the president's own relationship with his Chinese counterpart has frayed over the U.S.
Xi strongly supported Kim's meeting with Trump and hoped the two will find common ground, adding that China is ready to play a "positive" and "constructive" role in the process.
Mr Kim wraps up his visit - his longest trip to China - on Thursday.
Yonhap said Kim met with President Xi Jinping for about an hour on Tuesday and later attended a dinner at the Great Hall of the People in central Beijing hosted by Xi and his wife, Peng Liyuan. This time, Kim was accompanied by his wife, Ri Sol Ju, and North Korean Vice Chairman Kim Yong Chol, who is in charge of nuclear talks with the United States. "Kim also made the visit as a gesture that he wouldn't beg for an early North Korea-US summit", said Cha Du-hyeogn, a visiting scholar at Seoul's Asan Institute for Policy Studies. "China has actually been a good partner in our efforts to reduce the risk to the world from North Korea's nuclear capability; I expect they will continue to do so".
Relations between China and North Korea had deteriorated in recent years over Pyongyang's nuclear activities, but Mr Kim has made sure to keep Mr Xi informed about his dealings with the US and South Korea, and ties appear to have warmed.
In a New Year speech, Kim warned that Pyongyang may change its approach to nuclear talks if Washington persists with sanctions.
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