SEOUL, South Korea — A senior American negotiator arrived in North Korea on Wednesday to sort out crucial details for a nuclear summit meeting in Vietnam between President Trump and the North’s leader, Kim Jong-un, with only three weeks to go before the talks take place.

Stephen Biegun, the Trump administration’s special representative for North Korea, arrived in Pyongyang, the North’s capital, around the time that Mr. Trump announced in his State of the Union address that he and Mr. Kim would meet for a second time on Feb. 27-28 in Vietnam. Mr. Beigun’s trip had been announced in advance.

When Mr. Kim and Mr. Trump first met in Singapore in June, they agreed to work toward the “complete denuclearization of the Korean Peninsula” and to build “new” relations between their countries. But since then, talks have stalled over how to carry out that vaguely worded agreement.

Mr. Trump now wants “significant and verifiable progress on denuclearization, actions that are bold and real,” Mr. Biegun said last week in a speech at Stanford University. But American intelligence agencies recently cautioned that the North was “unlikely to completely give up its nuclear weapons and production capability.”