Iran’s supreme leader, Ayatollah Ali Khamenei, who has the final word on any nuclear deal, supported Iran’s nuclear negotiating team in a speech for the Iranian New Year while stressing there was no basis for broader discussions with the United States on regional issues.

But referring to one of the major impediments in the talks, he told an audience of thousands at a shrine in the northeastern city of Mashhad that sanctions must be lifted in one go, not in a step-by-step manner, as Iran fulfills its obligations under an accord, as the United States and its negotiating partners have proposed.

“If the other side has the ability to reimpose sanctions against the Iranian nation under any excuse, there is then absolutely no reason whatsoever for our negotiating team to accept a term which would be irreversible,” he said. Underscoring his demand for quick sanctions relief, Ayatollah Khamenei insisted Iran has remained faithful to its commitments in the talks.

“We did not break promises, we did not double-talk and we did not change our minds,” Ayatollah Khamenei said. “In contrast, the other side, the Americans, broke their promises, they changed their views and they cheated.”

Mr. Kerry’s meeting with his European counterparts was held at London’s Heathrow Airport and was arranged so that ideas could be exchanged on how to overcome the remaining issues in the talks, he said. The European participants included Laurent Fabius of France, Philip Hammond of Britain, Frank-Walter Steinmeier of Germany and Federica Mogherini, the European Union’s foreign policy chief, and their senior aides.

Toward the end of Mr. Kerry’s meeting, Mr. Hammond issued a statement that said that the United States and its negotiating partners agreed that any accord must ensure that Iran’s program be "exclusively peaceful."

Sergey V. Lavrov, the Russian foreign minister, and Wang Yi, China’s foreign minister, did not attend. But Mr. Kerry spoke with them by phone on Friday.