North Korea leader Kim Jong Un is in China for a fourth summit meeting with President Xi Jinping, North Korean and Chinese state media reported.

State-run Korea Central Television reported that Kim and his wife, Ri Sol Ju, are visiting China until tomorrow, at Xi's invitation.

The summit with Xi comes as Kim and President Donald Trump are negotiating over the location for a second summit meeting, and also came on Kim's 35th birthday yesterday.

Earlier, South Korea's Yonhap news agency reported that a North Korean train "possibly carrying a senior official" had crossed the border into China, passing through the Chinese border city of Dandong.

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Dozens of security vehicles and officials blocked the roads around the train station in Dandong before the train passed, Yonhap reported. The roads were reopened afterward.

Each of Kim's previous visits occurred before or after holding summits with Trump or South Korean President Moon Jae In. Kim is also expected to meet Moon again soon, possibly in Seoul.

Trump told reporters this week that negotiations were under way over a location for a second summit with Kim. And Trump said he had received a "great letter" from Kim.

South Korean newspaper Munhwa Ilbo reported that State Department officials recently met multiple times with North Korean counterparts in Hanoi to plan the summit, fuelling speculation that Vietnam could host the event.

"We are negotiating a location," Trump told reporters.

"It will be announced probably in the not-too-distant future," he said, according to the BBC. "They do want to meet and we want to meet and we'll see what happens."

Kim was also accompanied by senior officials including Vice-Chairman of the Central Committee of the Workers Party Kim Yong Chol and senior diplomat Ri Yong Ho, according to Korea Central Television.