Mr. Trump’s decision to accept Mr. Kim’s invitation to meet shocked the diplomatic establishment last week, coming after months of bellicose insults and threats between the two leaders and their governments. Any meeting, should it actually come to fruition, would probably be held by May, said Chung Eui-yong, a South Korean official who conveyed the invitation to the White House.

Mr. Ri was seen at the Beijing airport with Choe Kang-il, the deputy director general for North American affairs at the North Korean Foreign Ministry, Yonhap reported.

Sweden’s prime minister, Stefan Löfven, said on Saturday that he was willing to host a meeting between Mr. Trump and Mr. Kim.

“If we can help in any way, we will do it,” Mr. Löfven said at a news conference.

Mr. Löfven, in an interview with Sweden’s TT newswire, also cited his country’s role as a protecting power for the United States as a reason for acting as a conduit between it and North Korea.

“The fact that we are a protecting power for the U.S., have been at the border since the 1950s and have had an embassy in Pyongyang since the start of the 1970s has given us a relationship with North Korea in which we feel they trust us,” he said.

During Mr. Ri’s two-day trip to Stockholm, he will meet with Margot Wallstrom, Sweden’s foreign minister, the Swedish government said in a statement. The newspaper Dagens Nyheter said the talks would not include American or South Korean officials, but added that the United States and South Korea had been involved in preparations for the talks with Mr. Ri.

Mr. Ri’s trip to Sweden came as South Korea’s foreign minister, Kang Kyung-wha, traveled to Washington Thursday to meet with State Department officials with the aim of keeping a Trump-Kim meeting on track. The recent firing of Secretary of State Rex W. Tillerson has fueled fears that a changing of the guard at the State Department could derail plans for the talks.

“It is necessary to maintain close coordination at various levels in making preparations for critical diplomatic events going forward,” Ms. Kang said, alluding to relations between Washington and Seoul.

Ms. Kang was originally supposed to meet with Mr. Tillerson, but will instead meet with John Sullivan, the deputy secretary of state, who is now serving as acting secretary. Mr. Tillerson’s expected replacement, Mike Pompeo, is skeptical that negotiations with North Korea will lead it to give up its nuclear arsenal.

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