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President Trump said that the unprecedented meeting with Kim would take place by May, although a spokesman said Monday that the White House was "looking forward to a potential summit some months in advance".

South Korea says it's closely monitoring the arrival of a train in Beijing that looks like one used by North Korea's previous leader.

North Korean leader Kim Jong-un has reportedly left Beijing after his first trip outside the isolated state since he came to power in 2011, having travelled to the Chinese capital on his family's purpose-built armoured train.

Both Kim Jong Il and Kim Il Sung, the current leader's father and grandfather respectively, traveled by heavily armored train when they visited their allies in China and Russian Federation.

Despite the signs, a spokeswoman for the Chinese Foreign Ministry said she was not aware of the situation and had no further comment.

A visit by Mr Kim to Beijing would make sense against a broader backdrop of China trying to make sure that it plays a key role in any significant advance towards a denuclearised peninsula.

Relations between North Korea and China have been strained after Beijing imposed tougher sanctions on the hermit kingdom.

North Korean leader Kim Jong-un is visiting China in an unannounced trip, according to media reports, ahead of an inter-Korean summit and a US-North Korea summit.

Scott Uehlinger, a former Central Intelligence Agency station chief suggested the leader of the rogue state could be trying to win over China before his showdown meeting with President Donald Trump which is expected to take place in May.

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

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

Japanese television network NTV and public broadcaster NHK reported the arrival of the train and said the heavy security in the city suggested a senior official was aboard. There were reports of it transiting several stations on the way from North Korea to Beijing. The video did not show anyone getting off the train.

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. This is similar to Kim's predecessor Kim Jong-il. Since succeeding his father as leader in 2011, Kim has touted an image of his country as diplomatic equal to China and it's unlikely he would sneak into Beijing for his first face-to-face meetings with the Chinese leadership, the analysts said. They said it's more likely Kim sent a special envoy, possibly his sister Kim Yo Jong, to appease a traditional ally ahead of his planned meetings with the presidents of South Korea and the United States. The envoy could potentially seek Chinese commitment for future support should North Korea's talks with rivals fall through, said Du Hyeogn Cha, a visiting scholar at Seoul's Asan Institute for Policy Studies.

"If the talks with South Korea and the United States fall through, North Korea will surely try to demonstrate its nuclear weapons and missile capabilities again".

White House spokesman Raj Shah said Monday the USA could not confirm reports that Kim was visiting China.

"If North Korea speaks with the US on its own it might feel it is at a disadvantage, but if it has China as an ally, Pyongyang may think it will be able to protect its interests and profits during the summits", he said.