North Korean leader Kim Jong Un was spending his birthday in Beijing after arriving in the Chinese capital Tuesday — which was believed to be his 35th birthday — for his fourth summit with Chinese President Xi Jinping as talks on a second meeting with U.S. President Donald Trump gather steam.

“At the invitation of Xi Jinping, general secretary of the Central Committee of the Communist Party of China and Chinese president, Kim Jong Un, chairman of the Workers’ Party of Korea and chairman of the State Affairs Commission of the Democratic People’s Republic of Korea, is visiting China,” the official Xinhua News Agency quoted an unidentified spokesperson from the Chinese Communist Party’s Central Committee International Department as saying Tuesday.

Kim was to visit through Thursday, the spokesperson said.

The North’s official Korean Central News Agency also confirmed the visit, saying that Kim had 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.

Media reports said that a long motorcade believed to be carrying Kim and including motorcycle outriders reserved for state leaders left a train station in the Chinese capital shortly after the arrival of an armored train consisting of 20 to 25 cars — most of whose windows were blacked-out — along tracks lined by police and paramilitary troops.

Kim was expected to stay at the Diaoyutai State Guest House in the capital’s west, with meetings held at the Great Hall of the People, the seat of the legislature that sits next to Beijing’s Tiananmen Square.

While there was no official confirmation of Kim’s itinerary, South Korea’s Hankyoreh newspaper, citing an unidentified source with close knowledge of North Korea-China affairs, reported late Monday that Kim would meet with Xi.

After the visit’s announcement, Tokyo voiced hopes of receiving a briefing from Beijing on the trip.

“We’re making efforts to collect and analyze information with strong interest and hoping to have an appropriate briefing from the Chinese side from now on,” Kyodo News quoted Chief Cabinet Secretary Yoshihide Suga as saying at a news conference Tuesday.

Regarding the possible impact of Kim’s visit on the second summit with Trump, Suga said, “I’d like to refrain from prejudging at this point.”

Seoul on Tuesday said it hoped the North Korean leader’s trip to China would act as a “stepping stone” for the second Kim-Trump summit. South Korean presidential spokesman Kim Eui-kyeom made the comments hours after Kim arrived in Beijing, adding that he hoped the Xi-Kim talks would also help contribute to achieving the complete denuclearization and lasting peace on the Korean Peninsula.

Kim, who Seoul says was born on Jan. 8, 1984 — though the North’s government hasn’t officially confirmed the date — met with Xi in China three times last year before and after he held summits with Trump and South Korean President Moon Jae-in.

Beijing is Pyongyang’s sole major ally and its top economic lifeline as it continues to fend off crippling sanctions over its nuclear weapons and missile programs. The visits last year were widely seen as a courtesy to China and an opportunity to coordinate strategy ahead of the summits with the U.S. and South Korean leaders.

Xi is widely expected to visit North Korea at some point soon, possibly even this year — the 70th anniversary of diplomatic ties between the two countries — which would make him the first Chinese leader to do so since 2005.

Trump said Sunday that the sanctions imposed on the nuclear-armed North will remain “in full force and effect” until the United States saw “very positive,” but also said that discussions on the location of the next summit were underway and that further details would be announced soon.

“We are negotiating a location,” he said.

The White House has remained evasive on the exact timing of the summit, though officials have previously said they expected the meeting to happen sometime early this year.

“It will be announced probably in the not too distant future. They do want to meet and we want to meet and we’ll see what happens,” he said, adding the he had “indirectly spoken” with Kim.

On Monday, the South Korean newspaper Munhwa Ilbo reported that U.S. State Department officials recently met multiple times with North Korean counterparts in Hanoi and discussed planning a second summit between Trump and Kim, fueling speculation that Vietnam could host the event.

Trump, who held a landmark summit with Kim in Singapore in June that resulted in a vaguely worded pledge “to work toward the denuclearization of the Korean Peninsula,” said last week that he had received a “great letter” from the North Korean leader but declined to reveal its contents.

The latest letter from Kim came as denuclearization talks remain at an impasse and after the North Korean leader warned in a New Year’s speech that Pyongyang may change its approach to the talks if Washington persists with sanctions.

In his address, Kim urged the United States to take reciprocal measures in exchange for denuclearization steps the North Korean dictator has claimed his country has taken since last year.

“If the United States does not keep the promise it made in the eyes of the world, and out of miscalculation of our people’s patience, it attempts to unilaterally enforce something upon us and persists in imposing sanctions and pressure against our Republic, we may be compelled to find a new way for defending the sovereignty of the country and the supreme interests of the state,” Kim said.

Some observers believe that while it is tempting to interpret this “new way” as being a thinly veiled threat of more nuclear or missile tests, it could instead be an implicit warning that the North could further bolster its already improving ties with China if the U.S. fails to ante up.

“Kim’s confidence stems from the expectation of growing and reliable support by China,” Rudiger Frank, a professor of East Asian economy and society at the University of Vienna, wrote on the North Korea-watching 38 North blog on Jan. 2. “The three summits with Xi Jinping in 2018 seem to have made Kim Jong Un very optimistic.”

Frank said the protracted trade war between Beijing and Washington, among other issues, has created the impression among strategists in Pyongyang of a “Cold War 2.0 situation.”

“Like in the decades before the collapse of the Soviet Union, supporting smaller allies could again become a matter of principle for the Big Powers even if these allies step out of line occasionally,” he wrote. “The not unfounded hope of Kim Jong Un is that in such a strategic setting, China would be willing to provide protection and economic support while abstaining from too massive direct interference. Forcing the U.S. out of Korea, and out of East Asia, is more important to Beijing than reigning in on a self-confident or even provocative North Korea.”

Thus, Kim’s threat may not be a hint at more nuclear tests, “but rather as a message to Donald Trump: You are not our only option for security and economic development. If you refuse to be cooperative, we will ignore you and turn to China. Oh, and we will take South Korea along,” he added.

Still, the North Korean leader is already refraining from putting all of his eggs in one basket, saying in his New Year’s address that, despite the stalled talks, he was willing to meet Trump again at any time to produce results “welcomed by the international community.”