BEIJING/SEOUL (REUTERS, AFP) - North Korean leader Kim Jong Un is visiting China at the invitation of Chinese President Xi Jinping, only days after warning he may take an alternative path if the United States does not ease sanctions and pressure on his isolated country.

Kim arrived at Beijing train station on Tuesday (Jan 8) aboard his distinctive dark green train, emblazoned with a yellow stripe at 10.55am local time, South Korea's Yonhap news agency reported. .

The visit, confirmed by North Korean and Chinese state media, will likely lead to Mr Kim's fourth summit with Xi in the last year and comes amid plans for a second summit with US President Donald Trump aimed at denuclearising the Korean peninsula.

Mr Kim held three summits with Mr Xi, his most important ally, last year, before and after summits with Mr Trump and South Korean President Moon Jae-in.

"Both Xi and Kim see value in coordinating their positions in advance of Trump-Kim summits. That appears to be a pattern," Dr Bonnie Glaser, a senior adviser at the Centre for Strategic and International Studies, told AFP.

"Kim also seeks Beijing's help in getting international sanctions eased."

While China and Russia have said the United Nations should consider relaxing sanctions on North Korea, Mr Trump insisted on Sunday that they would remain "in full force and effect" until the US sees "very positive" results in the nuclear issue.

"Kim is eager to remind the Trump administration that he does have diplomatic and economic options besides what Washington and Seoul can offer," Mr Harry Kazianis, director of defence studies at US-based Centre for the National Interest said in an e-mailed statement.

"In fact, during his New Year's Days speech, Kim's 'new way' that he referred to may well have been a veiled threat to move closer to Beijing. That should make America quite concerned."

Mr Kim's trip also coincides with the second day of talks between US and Chinese officials in Beijing aimed at resolving their trade war - China has in the past rejected the notion that it was using the North Korean issue as a bargaining chip in the negotiations.

"Xi also gains from a summit with Kim - and the timing could not be any better," said Mr Kazianis. "With Chinese and US officials meeting to discuss how to end the growing trade war between the two superpowers, it shows Beijing clearly has a North Korea card to play if it sees fit."

Mr Kim left for China on a private train on Monday afternoon accompanied by his wife, Ri Sol Ju, and other senior North Korean officials, including Kim Yong Chol and Ri Yong Ho, North Korea's state-run KCNA news agency said. China's official Xinhua news agency also confirmed that Mr Kim is visiting from Monday to Thursday at Mr Xi's invitation.

"He was warmly seen off by leading officials of the Party, government and armed forces organs at the railway station," KCNA said in its report.

Mr Kim's visit of North Korea's most important economic and diplomatic ally, which was first reported by South Korean media, comes amid reports of advanced negotiations for a second summit between Washington and Pyongyang aimed at resolving the standoff over North Korea's nuclear programme.

Mr Kim said in a New Year speech last week he is ready to meet Mr Trump anytime to achieve their common goal of denuclearisation of the Korean peninsula. But he warned that he may seek an alternative path to a summit with Mr Trump if US sanctions and pressure against the country continues.

While there were no details released about the possible agenda, Mr Kim has been seeking relief from international sanctions, a peace declaration to formally end the 1950-1953 Korean War, and more economic investment.

Ties between China and North Korea, which had frayed as Pyongyang stepped up its provocations through a series of missile and nuclear tests, warmed over the last year as Mr Kim engaged with Beijing as well as Seoul and Washington.

Neither KCNA nor Xinhua provided further information on Mr Kim's itinerary, though South Korea's Hankyoreh newspaper said on Monday that he will meet Mr Xi for a fourth summit.

None of Mr Kim's 2018 trips to China were announced in advance, and the earliest indications of the first one came when his train was spotted in Beijing.

In the Chinese border city of Dandong, dozens of security vehicles and officials blocked the roads around the train station on Monday before the train passed through, Yonhap reported, before reopening them afterwards. It had not been confirmed whether a senior North Korean official was on board, it quoted its source as saying.

But Japan's Kyodo news agency said hotel guests in Dandong had not been allowed to enter rooms facing the river that forms the border on Monday afternoon, in what it cited sources describing as "an apparent move to prevent the train from being seen".

Mr Kim's birthday is reportedly on Tuesday, although that has never been confirmed by Pyongyang.

In their first summit in Singapore last June, the two men signed a vaguely-worded pledge about the denuclearisation of the Korean peninsula, but progress has since stalled amid disagreements over what that means, with meetings and visits cancelled at short notice.

Negotiations were underway on the location of their next meeting, Mr Trump said on Sunday, while remaining evasive on its timing. The US President said last week he had received a "great letter" from the North Korean leader but declined to reveal its contents.

"We are negotiating a location," he told reporters. "They do want to meet and we want to meet and we'll see what happens."

Washington is demanding Pyongyang gives up its nuclear arsenal before any relief from economic sanctions is granted, while the North is insisting on immediate concessions from the US.

Culminating in late 2017, Pyongyang carried out six atomic blasts and launched rockets capable of reaching the entire US mainland, but has now carried out no such tests for more than a year.

Officials from the Blue House in Seoul could not immediately be reached for comment by AFP.