There are reports that North Korea leader Kim Jong-un is visiting China in what would be his first overseas trip since taking power in 2011, and ahead of a potential summit with US President Donald Trump.

Bloomberg cited three unnamed sources confirmed the visit, while earlier, Japanese media reported a high-ranking North Korean official arrived by train in Beijing.

The Kyodo News Agency, citing sources close to the matter, said the visit of the official was intended to improve ties between Beijing and Pyongyang .

Relations between the two states have been frayed by North Korea's pursuit of nuclear weapons and China's backing of tough sanctions against North Korea at the United Nations Security Council.

Reuters was unable to immediately confirm that a top North Korean official had arrived by train.

Footage from Nippon News Network showed what an announcer described as a green train carriage with yellow horizontal lines as part of a 21-car train.

It is similar to the kind that Mr Kim's late father, Kim Jong-il, travelled on when he visited Beijing in 2011.

Beijing has traditionally been North Korea's closest ally.

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.

Kyodo, citing sources, reported that on Sunday, a special train that might have carried the official passed through the Chinese border city of Dandong.

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

The rail journey between Dandong and Beijing covers more than 1,100km and takes at least 14 hours by ordinary service, according to Chinese railway timetables.

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 Mr Kim was passing through.

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

Police also cleared out 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 yesterday evening.

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.

The Beijing railway bureau also warned on its microblog, without giving a reason, of multiple train delays of up to two hours in the Beijing region.

A source with ties to the Chinese military told Reuters that it was "not possible to rule out the possibility" that Mr 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.

Other diplomatic sources said they were aware of the speculation that Mr Kim was visiting but were not able to immediately confirm it.

Visits to China by Kim Jong-ill were only confirmed by both China and North Korea once he had left the country.

Kim Jong-il travelled by private train during his rare visits to China or Russia under tight security. Diplomats and other sources have said Kim Jong-il avoided flying for overseas trips due to security concerns.

The younger Kim, who was educated in Switzerland, is not known to have any fear of flying and state media have shown pictures of him aboard a plane.

However, he is not known to have travelled outside the country since assuming power in late 2011 after his father's death.