Kim Jong-un on 4-day visit to China to meet President Xi: Report

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.

China is the isolated, nuclear-armed North's key diplomatic ally and main source of trade and aid, and the visit is likely to heighten speculation about a potential second summit between Kim and US President Donald Trump.

Kim - whose birthday is reportedly on Tuesday, although that has never been confirmed by Pyongyang - visited China three times last year to pay his respects to Xi.

Until his first trip in March, Kim had not met 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 was 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 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.

Japan's Kyodo news agency added that hotel guests in Dandong had not been allowed to enter rooms facing the river that forms the border on Monday afternoon, in what it cited sources describing as "an apparent move to prevent the train from being seen".

Officials from the presidential Blue House in Seoul were said to be monitoring the situation.

At the Singapore summit, Kim and Trump signed a vaguely-worded 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 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.

Culminating in late 2017, Pyongyang carried out six atomic blasts and launched rockets capable of reaching the entire US mainland, but has now carried out no such tests for more than a year.