North Korean leader Kim Jong-un may be on a secret visit to China in what would be his first known overseas trip since taking power in 2011, news reports said.

Bloomberg cited three unnamed sources as saying on Monday that Kim was in China, but they gave no other details. They requested anonymity because of the sensitivity of the situation.

Japan's Kyodo news agency, also citing unidentified sources, reported on Sunday that a special train that might have carried the North Korean leader passed through the Chinese border city of Dandong.

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

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

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

Kyodo, citing sources close to the matter, said the visit was intended to improve ties between Beijing and Pyongyang, which have been frayed by North Korea's pursuit of nuclear weapons and China's backing of tough sanctions at the UN Security Council.

'Monitoring the situation'

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

Asked at a daily briefing about the reports, Chinese Foreign Ministry spokeswoman Hua Chunying said on Monday she was unaware of the situation.

South Korea gave no details about a possible trip either.

"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.

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

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

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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 on Monday evening.

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

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

The younger Kim, who was educated in Switzerland, is not known to have travelled outside the country since assuming power in late 2011 after his father's death.