North Korean leader Kim Jong-un has arrived in China for a four-day visit at the invitation of Chinese President Xi Jinping, as preparations increase for a second summit with US President Donald Trump.

The Korean Central News Agency said on Tuesday that Kim departed on Monday afternoon on a private train accompanied by his wife Ri Sol Ju and top North Korean officials.

A motorcade late on Tuesday morning was seen leaving a train station in the Chinese capital, Beijing, with police blocking a street with metal gates hours after Kim's train crossed the border into northern China, according to media reports.

During his stay in China, North Korea's key diplomatic ally and main source of trade and aid, Kim is expected to hold his fourth summit with Xi.

The visit comes a week after Kim warned that North Korea may seek an alternative course if the United States maintains sanctions and pressure on his country.

Last year, Kim travelled to China three times to meet Xi before and after meeting Trump as well as summits with South Korean President Moon Jae-in.

"What's interesting about Kim's visits over the past year is that they either preceded or occurred after major diplomatic moves," Al Jazeera's Divya Gopalan, reporting from Hong Kong, said.

"So this time it seems that he wants to involve Xi Jinping and China in that process," she added. "China, of course, wants to be front and centre with any moves between North Korea and the US."

Analysts also believe that Kim is eager to use the fact that relations between China and the US are strained amid the world's two biggest economies' bitter trade war, in order for North Korea to get as much as possible out of the expected talks, Gopalan noted.

"Involving China would be a key strategic move for North Korea," she said.

The visit also coincided with what South Korean officials say is Kim's 35th birthday on January 8.

"He was warmly seen off by leading officials of the Party, government and armed forces organs at the railway station," KCNA said in its report.

China's role

The latest trip comes after US and North Korean officials are believed to have met in Vietnam to discuss the location of a second summit between Kim and Trump aimed at denuclearising the Korean Peninsula.

At the Singapore summit, the two leaders signed a vaguely-worded pledge on denuclearisation, but progress has since stalled with Pyongyang and Washington arguing over their agreement's interpretation.

In his annual New Year's address last week, Kim renewed his commitment to denuclearisation but added that the progress would be faster if Washington took the corresponding action.

North Korea would have "no option but to explore a new path in order to protect our sovereignty" if the US "miscalculates our people's patience, forces something upon us and pursues sanctions and pressure without keeping a promise it made in front of the world", Kim said, adding that he was ready to meet Trump again at any time.

Christopher Hill, a former US ambassador to South Korea, said Kim's visit to China may be Beijing's way of ensuring it remains a player in any future developments with Washington.

"With the Trump administration, the [US's] relationship with China has really gone very sour, and moreover as the Trump administration ramped up its North Korean contacts, they did that without an effort to bring China in - and China is a very substantial country with 1.4 billion people and they really don't want to be ignored," he told Al Jazeera.

"So I think this is China's effort to try and be part of this process, and the key question, of course, is what is their message to the North Koreans?" Hill added.

"Are the Chinese going to encourage the North Koreans to do more? Or is North Korea going to continue a policy which essentially involves not testing any weapons during this time but hasn't really involved much else."