A mysterious 21-carriage train arriving in Beijing amid heightened security has sparked intense speculation that Kim Jong Un, the North Korean leader, is visiting the city in what would be his first overseas trip since taking power.

Bloomberg reported that Kim had arrived in the Chinese capital, citing three unnamed sources, in the latest round of diplomatic activity ahead of a potential summit with US President Donald Trump.

Japanese media said only that a high-level official had arrived. Kyodo, citing sources close to the matter, said the visit was intended to improve ties between Beijing and Pyongyang that have been frayed by North Korea's pursuit of nuclear weapons and China's support for tough sanctions against North Korea at the United Nations.

Footage from Nippon News Network showed what an announcer described as a green train carriage with yellow horizontal lines, part of a 21-carriage train, similar to the kind that Kim's late father, Kim Jong Il, rode when he visited Beijing in 2011.

Beijing has traditionally been secretive and isolated North Korea's closest ally, but Kim is due to hold summit meetings separately with its rivals South Korea and the United States.

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Asked earlier at a daily news briefing about reports of an important North Korean visitor arriving at the Chinese border city of Dandong, Chinese Foreign Ministry spokeswoman Hua Chunying said she was unaware of the situation.

"The Government is closely communicating with relevant countries and monitoring the situation," South Korea's presidential Blue House said in a statement via a messaging app.

Two sources in northeastern China also told Reuters that a North Korean visitor had crossed into Dandong by train.

On Chinese social media some residents of Dandong said that there had been high security around the train station there and said there were rumours that Kim was passing through.

Police tightened security along Beijing's main east-west thoroughfare, Changan Avenue, closing off the entrances to some of the buildings which face the road.

A military honour guard is seen marching away after a convoy of vehicles enter the the Diaoyutai State Guesthouse where top North Korean leaders have been known to stay on previous trips to Beijing.
A military honour guard is seen marching away after a convoy of vehicles enter the the Diaoyutai State Guesthouse where top North Korean leaders have been known to stay on previous trips to Beijing.
Police officers guard an intersection near the Diaoyutai State Guesthouse.
Police officers guard an intersection near the Diaoyutai State Guesthouse.

Police also cleared all tourists from Tiananmen Square around the same time, which normally only happens when important meetings are happening in the Great Hall of the People, where top Chinese leaders often meet visiting heads of state.

There was a large security presence outside the Great Hall.

Reuters reporters saw a lengthy motorcade, including a limousine with dark tinted windows, heading down Changan Avenue in the direction of the Diaoyutai State Guest House and away from the Great Hall of the People, flanked by a police escort on motor-bikes.

A source with ties to the Chinese military told Reuters that it was "not possible to rule out the possibility" that Kim was visiting Beijing, but cautioned this was not confirmed.

A diplomatic source told Reuters that there was heavy security around the Diaoyutai State Guest House, where some high level foreign visitors stay during visits to the city.