SEOUL—The U.S. special envoy for North Korea concluded three days of nuclear-disarmament talks in Pyongyang ahead of the , scheduled for this month in Vietnam, expressing confidence that “real progress” was possible if both sides remain committed.

Stephen Biegun, one of Washington’s top nuclear negotiators, held talks with North Korean officials from Wednesday through Friday, the first public visit to Pyongyang by a senior U.S. official since October. The two sides discussed the North’s “complete denuclearization,” advancing...