North Korea's leader Kim Jong-un and his wife Ri Sol-ju are featured in this photo released by the Korean Central News Agency, Tuesday. The state media said Kim was visiting China from Monday to Thursday at Xi's invitation. KCNA-Yonhap

North Korean leader Kim Jong Un arrived in Beijing on Tuesday at the start of a four-day visit, in what's likely an effort to coordinate with his only major ally ahead of a summit with U.S. 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 armored train consisting of 20 to 25 cars ― most of whose windows were blacked-out ―along tracks lined by police and paramilitary troops.

Kim's trip, announced earlier by both sides, comes after U.S. 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 Kim departed Monday afternoon with his wife Ri Sol Ju and other top officials. It said Kim is visiting China at the invitation of Chinese President Xi Jinping. Tuesday also happens to be Kim's birthday.

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.

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 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 Kim's assumption of power 2011, ties have improved remarkably over the past year as Xi seeks to maintain his influence in the region.

Kim's trip comes as the U.S. and North Korea look to settle the North's decades-long pursuit of a nuclear arsenal.

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 tantalizingly close to its nuclear goal of one day targeting with pinpoint accuracy anywhere on the U.S. mainland.