FILE - People watch a screen showing images of, from left, South Korea's president Moon Jae-in, US president Donald Trump, China's president Xi Jinping, and North Korea's leader Kim Jong Un at a railway station in Seoul, May 11, 2018.
Kim was invited by Xi and accompanied by his wife Ri Sol Ju and several top officials on his train journey across the border, the state-run Korean Central News Agency said.
Kim's first foray outside of North Korea as leader was for a summit with Xi last March.
The Kim-Xi meeting coincides with talks this week between a USA delegation and Beijing to de-escalate the ongoing trade war between China and the U.S.
Diplomatic sources in Beijing said that Kim's fourth trip to China came at the request of the North Korean leader and that Pyongyang had initially requested a visit in December but that it was pushed to early January because of Xi's tight domestic schedule.
Trump has repeatedly said he will meet with Kim again in the near future, but the two sides have failed to agree on anything significant enough to hold another summit yet. "The US is very clear about China's position", he said, reaffirming China's commitment to reach a trade deal with US benefitting both the countries.
Chinese foreign ministry spokesman Lu Kang, asked during a daily briefing whether there was a connection between Kim's visit and the possibility of another meeting between Kim and Trump, said China believed contact between the United States and North Korea was important. But he warned that he may seek an alternative path to a summit with Trump if United States sanctions and pressure against the country continues.
Kim's trip comes as the USA and North Korea look to settle the North's decades-long pursuit of a nuclear arsenal. But he said Beijing remains supportive of efforts to end tensions over US demands for a halt to North Korea's nuclear and missile programs.
Providing the bulk of both North Korea's diplomatic support, trade and aid.
The visit coincided with what South Korean officials say is Kim's 35th birthday on Tuesday.
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Kim made two visits to China before his June 12 summit with Trump, and one after the Singapore summit.
Officials from USA and China are meeting in Beijing this week to negotiate an end to the trade war that has led to hundreds of billions of dollars in tariffs over the past year.
However, Washington-based Center for Strategic and International Studies senior adviser Bonnie Glaser said that the timing of the North Korean leader's arrival could be coincidental.
"Supreme interests of the state", he added, quoting Kim's speech, "means fulfilling the great national task", the state mission of the North: "Unify Korea under Kim and his ruling Workers' Party of Korea".
Washington is demanding Pyongyang give up its nuclear arsenal before any relief from economic sanctions is granted, while the North is insisting on immediate concessions from the US.
"We hope that the latest exchange between China and the North would help establish complete denuclearisation and permanent peace on the Korean peninsula and serve as a springboard for a second North-US summit", he told reporters.
US and Chinese officials are also holding trade talks in Beijing this week after agreeing on a temporary truce to their trade war last month.
In May 2018, Trump once chose to cancel his planned talks with Kim, saying he detected North Korea starting to take a tougher stance against the United States in the wake of the second summit between Beijing and Pyongyang earlier in the month.
"Their behaviour has demonstrated that as well and we appreciate that", Pompeo said. "China's ability to use [North Korea] as leverage has diminished considerably since Trump opened his own channel to Kim". "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".