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.

The visit, confirmed by North Korean and Chinese state media, will likely lead to Kim’s fourth summit with Xi in the last year and comes amid plans for a second summit with Trump aimed at denuclearising the Korean peninsula. Kim held three summits last year with Xi, his most important ally, before and after summits with Trump and South Korean President Moon Jae-in.

“Kim is eager to remind the Trump administration that he does have diplomatic and economic options besides what Washington and Seoul can offer,” Harry J. Kazianis, Director of Defence Studies at U.S.-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.”

Kim left for China on a private train on Monday afternoon accompanied by his wife, Ri Sol Ju, and 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 Kim is visiting from Monday to Thursday at Xi’s invitation. The visit coincided with what South Korean officials say is Kim’s 35th birthday on January 8.

“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.

Kim’s visit to 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.

Kim said in a New Year speech last week he is ready to meet 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 Trump if U.S. sanctions and pressure against the country continues.

In an interview with CNBC on Monday, U.S. Secretary of State Mike Pompeo praised China’s support for resolving the North Korean crisis and said he did not think the U.S. trade dispute with Beijing would affect this. “The Chinese have been very clear to us that these are separate issues,” Pompeo said.

“Their behaviour has demonstrated that as well and we appreciate that. China has actually been a good partner in our efforts to reduce the risk to the world from North Korea’s nuclear capability; I expect they will continue to do so.”

South Korea’s foreign ministry said it was aware of Kim’s planned visit and hopes Kim’s latest visit and summit with Xi would contribute to the shared “strategic goal” of achieving complete denuclearisation and lasting peace on the Korean peninsula.

While there were no details released about the possible agenda in China, 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 Kim engaged with Beijing as well as Seoul and Washington.

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