BEIJING — Kim Jong Un is on a four-day visit to China to meet President Xi Jinping, Chinese and North Korean state media said Tuesday, after a special train was reported to have been seen crossing the border.

The North’s leader, accompanied by his wife Ri Sol-ju and several senior officials set off from Pyongyang station on board his private train on Monday, the Korean Central News Agency reported.

China’s official Xinhua news agency also confirmed the visit.

The trip is at Xi’s invitation and set to run over four days, KCNA added, with Kim slated to return on Thursday.

The visit is likely to heighten speculation about a potential second summit between Kim and US President Donald Trump.

Kim visited China three times last year. Until his first trip in March, Kim had not meet Xi for more than six years after inheriting power from his father, as relations between the neighbours, once described as close as "lips and teeth", deteriorated.

But a whirlwind of diplomacy enveloped the Korean peninsula last year, with Kim also meeting the South’s President Moon Jae-in three times, and culminating in his high-profile Singapore summit with US counterpart Donald Trump in June.

None of Kim’s 2018 trips to China were announced in advance, and the earliest indications of the first one -- before Kim met either Moon or Trump -- came when his train was spotted in Beijing.

The South’s Yonhap news agency cited an informed source late on Monday saying that a special North Korean train had been seen crossing the border and was expected in Beijing Tuesday morning.

In the Chinese border city of Dandong, dozens of security vehicles and officials blocked the roads around the train station on Monday before the train passed through, Yonhap reported, before reopening them afterwards.

At the Singapore summit Kim and Trump signed a pledge about the denuclearisation of the Korean peninsula, but progress has since stalled amid disagreements over what that means, with meetings and visits cancelled at short notice.

Negotiations were underway on the location of their next meeting, Trump said on Sunday, while remaining evasive on its timing.

The US president said last week he had received a "great letter" from the North Korean leader but declined to reveal its contents.

"We are negotiating a location," he told reporters. "They do want to meet and we want to meet and we’ll see what happens."

Washington is demanding Pyongyang gives up its nuclear arsenal before any relief from economic sanctions is granted, while the North is insisting on immediate concessions from the US. – AFP