The train crossed the border around 10.15 pm local time (1315 GMT) and was expected in the Chinese capital Tuesday morning, the Southern news agency reported, citing an informed source.

China is the isolated, nuclear-armed North's key diplomatic ally and main source of trade and aid, and Kim visited three times last year to pay his respects to leader Xi Jinping.

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.

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.

It had not been confirmed whether a senior North Korean official was on board, it quoted its source as saying.

But Japan's Kyodo news agency said 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".

Kim's birthday is reportedly on Tuesday, although that has never been confirmed by Pyongyang, and a visit by him to China would be likely to heighten speculation about a second Kim-Trump summit.

In Singapore the two men 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.

Officials from the Blue House in Seoul could not immediately be reached for comment by AFP.