North Korean leader Kim Jong Un has arrived in Beijing at the start of a four-day visit.

The trip is likely to be an effort to coordinate with his only major ally ahead of a summit with US president Donald Trump that could happen early this year.

A long motorcade including motorcycle outriders reserved for state leaders left a Beijing train station shortly after the arrival of an armoured train consisting of 20 to 25 cars — most of whose windows were blacked out — along tracks lined by police and paramilitary troops.

Mr Kim’s trip, announced earlier by both sides, comes after US and North Korean officials are believed to have met in Vietnam to discuss the location of a second summit.

North’s Korean Central News Agency said Mr Kim departed on Monday afternoon with his wife Ri Sol Ju and other top officials.

It said Mr Kim is visiting China at the invitation of Chinese President Xi Jinping.

Tuesday also happens to be the reclusive North Korean leader’s birthday.

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A train similar to one seen during previous visits by Kim Jong Un arrives at Beijing Railway Station (AP/Mark Schiefelbein)

China’s official Xinhua News Agency issued a nearly-identical report, while Beijing’s North Railway Station was cocooned in security, with dozens of police and paramilitary troops patrolling outside.

Mr Kim is expected to stay at the highly-secure Diaoyutai State Guest House in the capital’s west, with meetings held at the Great Hall of the People, the hulking seat of the legislature that sits next to Tiananmen Square.

The trip marked a further break with past practice in that it was announced in advance of Mr Kim’s arrival, a possible sign of growing confidence on the part of North Korea and China, Pyongyang’s most important trading partner and a key buffer against pressure from Washington.

After years of cool relations following Mr Kim’s assumption of power 2011, ties have improved remarkably over the past year as Mr Xi seeks to maintain his influence in the region.

Mr Kim’s trip comes as the US and North Korea look to settle the North’s decades-long pursuit of a nuclear arsenal.

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A limousine similar to one Kim Jong Un has used previously is seen leaving the train station (AP/Ng Han Guan)

Washington and Pyongyang seemed close to war at points during 2017, as the North staged a series of increasingly powerful weapons tests that got it tantalisingly close to its nuclear goal of one day targeting with pinpoint accuracy anywhere on the US mainland.

Possibly fearing the economic effect of crushing outside sanctions imposed because of his weapons’ tests, Mr Kim abruptly turned to diplomacy with Seoul and Washington last year.

He also visited China three times, notably without a reciprocal visit from Mr Xi in a break with diplomatic convention.

But even after what was seen as a blockbuster summit between Mr Kim and Mr Trump in Singapore last June — the first-ever between the leaders of the war enemies — there has been little real progress in nuclear disarmament.

Washington is pressing the North to offer up a detailed accounting of its nuclear arsenal, while Pyongyang says it has already done enough and it is time for the US to ease harsh international sanctions that hold back the North Korean economy.

Mr Trump has offered assurances that another summit will allow he and Mr Kim to make a grand deal to settle the nuclear standoff and change a relationship marked by decades of animosity and mistrust.

However, outside analysts are highly sceptical that the North will easily abandon a nuclear arsenal constructed in the face of deep poverty and likely seen by Mr Kim as his only guarantee of regime survival.

Instead, Mr Kim may be seeking to gauge China’s attitude toward sanctions ahead of the talks, including what the North would have to concede in order to win Beijing’s support at the UN.

The North has held off on additional nuclear weapons and missile tests for more than a year, possibly in response to China’s displeasure, while carrying out its new diplomatic offensive.