Increased security at the border crossing of Dandong had tipped off South Korean media that Kim was likely headed back for another meeting with Xi Jinping.
The North's official Rodong Sinmun reported on its front page Tuesday that Kim, accompanied by his wife Ri Sol-ju and top aides, kicked off a four-day visit to China Monday at the invitation of Xi. Notably, Kim would be spending what is presumably his 35th birthday on January 8 in China.
Kim's first foray outside of North Korea as leader was for a summit with Xi last March.
Observers say the outcome of Xi- Kim meeting would be watched for its likely impact on the planned second summit meet between Kim and Trump.
South Korea yesterday welcomed Mr Kim's trip to China with a statement from the Blue House, the presidential palace, saying it will be a "stepping stone towards a second North-US summit".
"Kim is eager to remind the Trump administration that he does have diplomatic and economic options besides what Washington and Seoul can offer", Harry J. Kazianis, Director of Defence Studies at the US -based Centre for the National Interest, said in an e-mailed statement.
Chinese foreign ministry spokesman Lu Kang, asked during a daily briefing whether there was a connection between the trade talks and the possibility of another meeting between Kim and Trump, said China believed contact between the United States and North Korea was important. Both visited China and South Korea six years after inheriting power, with the elder Kim creating what Sung-Yoon Lee, a Korea expert at The Fletcher School at Tufts University in the USA, characterized as " a dramatic mood swing in Northeast Asia" - a plan Kim Jong Un appears set on emulating. Since then, "North Korea has refused to engage in working-level talks with the U.S.", NPR has reported.
United States diplomats in Beijing have been seriously monitoring interactions between China and North Korea, a source familiar with the matter told Kyodo News.
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Chinese officials are unhappy with United States curbs on exports of "dual use" technology with possible military applications. Its one-sentence announcement gave no other details of the agenda or who else would take part.
In his address, Kim urged the United States to take reciprocal measures in exchange for denuclearization steps the North Korean dictator has claimed his country has taken since previous year. However China's state-run Global Times newspaper appeared to reveal the birth year of the North Korean leader in a tweet.
But even after what was seen as a blockbuster summit between Kim and Trump in Singapore last June - the first ever between the leaders of the war enemies - there's been little real progress in nuclear disarmament.
Mr Trump has offered assurances that another summit will allow he and Mr Kim to make a grand deal to settle the nuclear standoff and change a relationship marked by decades of animosity and mistrust.
Washington is pressing North Korea to offer up a detailed accounting of its nuclear arsenal, while the North says it has already done enough and it's time for the U.S.to ease the harsh worldwide sanctions that hold back the North Korean economy.
The only day North Korea has acknowledged Mr Kim's birthday was in 2014, when visiting basketball star Dennis Rodman sang "Happy Birthday" to him after an exhibition match in Pyongyang.
Kim's visit comes as a United States delegation opened a second day of talks with Chinese counterparts in Beijing aimed at ending the bitter trade war between the world's two largest economies.
As talks with the United States stall over ending North Korea's nuclear weapons program, a question hangs: Will he and Trump meet again?
The North has held off on additional nuclear weapons and missile tests for more than a year, possibly in response to China's displeasure, while carrying out its new diplomatic offensive. "The US is very clear about China's stance".
The meeting with Xi could have a big impact on Trump's efforts to get Kim to abandon his hard-won nuclear arsenal.