North Korea’s ruler, Kim Jong-un, has arrived in Beijing at the invitation of Xi Jinping, seemingly to coordinate their countries’ approach for a summit with Donald Trump that could happen early this year.

Kim’s visit, which state media said would last four days, is his fourth time in China in the last year. It comes amid reports of advanced negotiations for a second summit between the North Korean leader and the US president.

Denuclearisation talks between the North and the US have stalled after a landmark meeting in Singapore last year, but speculation has mounted of a potential second summit between Kim and Trump. Kim’s visit comes after US and North Korean officials reportedly met in Vietnam to discuss the location of a second summit.

“One important reason for him to visit China is to strategise with China on these issues beforehand,” said Lu Chao, the director of the Korea Research Center at the Liaoning Academy of Social Science.

On Monday, Kim’s armoured train was spotted entering China. The North Korean leader, his wife Ri Sol-ju and several senior officials were onboard, the official Korean Central News Agency reported. On Tuesday morning, a long motorcade left a Beijing train station after the arrival of what appeared to be Kim’s train. Dozens of police and paramilitary troops were patrolling the station.

The South Korean presidential spokesman Kim Eui-kyeom said on Tuesday Seoul hoped the trip to China would act as a “stepping stone” for a second Trump-Kim summit. A Chinese foreign ministry spokesperson said Beijing supported contact between the US and North Korea and a resolution of the Korean peninsula issue.

The US has been pushing North Korea to disclose further details about its nuclear programme while Pyongyang has called on the US to relax the international sanctions crippling the North Korean economy.

During a New Year’s Day speech, Kim said he was ready to meet Trump, but warned his country may still seek an alternative path if US sanctions and pressure against North Korea continued.

Harry J Kazianis, the director of defence studies at the US-based Centre for the National Interest, said: “Kim is eager to remind the Trump administration that he does have diplomatic and economic options besides what Washington and Seoul can offer.”

Kim visited China three times during 2018 to see the Chinese president after not meeting him for more than six years after succeeding his father, Kim Jong-il. Analysts say the frequency of visits signal efforts to repair ties between Pyongyang and its closest ally.

None of Kim’s previous trips were announced in advance and the earliest indications of the first – in March, before Kim had met either South Korea’s president, Moon Jae-in, or Trump – came when his private train was spotted in Beijing.

In contrast, Kim’s latest visit was confirmed by state media in both countries ahead of his arrival in Beijing. The trip, to last until Thursday, is also longer than his previous visits, according to Lu.

Few details about Kim’s itinerary have been released. North Korean state media said Kim would be spending his 35th birthday, on Tuesday, in Beijing. He is expected to stay at the Diaoyutai State Guest House, which is reserved for visiting dignitaries.

Observers say Kim may be trying to gauge China’s support for removing sanctions. China, a permanent member of the UN security council, has agreed to previous rounds of sanctions against Pyongyang. Recently Chinese officials have called on the UN to consider relaxing them.

Trump insisted on Sunday they would remain “in full force and effect” until the US saw “very positive” results over the nuclear issue.

Culminating in late 2017, Pyongyang carried out six nuclear tests and launched rockets capable of reaching the entire US mainland. It has halted such tests for more than a year and has engaged in a diplomatic campaign.

Bonnie Glaser, a senior adviser at the Centre for Strategic and International Studies, said: “Both Xi and Kim see value in coordinating their positions in advance of Trump-Kim summits. That appears to be a pattern. Kim also seeks Beijing’s help in getting international sanctions eased.”