Mr. Vance said earlier at a press briefing that he did not know if he would travel to Switzerland on Friday for the talks, where he was initially expected for a signing ceremony.

Andy Burnham, Labour’s most popular politician, beat the populist right-wing Reform U.K. party in the Makerfield by-election. The decisive victory will energize his bid to become prime minister.

The vice president said the United States had leverage to dictate the outcome of the next round of negotiations. But he claimed incorrectly that Iran got no new benefit from the lifting of oil sanctions.