Negotiators from Iran and a group of six world powers meet again Wednesday in Geneva, seeking to build on their talks two weeks ago and reach a deal in a decade-long standoff over Iran's nuclear program.
The five permanent members of the U.N. Security Council, along with Germany, want an interim agreement that calls for Iran to stop some of its enrichment activity and accept more inspections in return for limited sanctions relief.
Iran's supreme leader Ayatollah Ali Khamenei said Wednesday that Iran will not retreat on its nuclear rights and that he set "red lines" for his negotiators.
Ahead of the talks, both sides expressed hope for working out a deal.
Iran's foreign minister and chief nuclear negotiator Javad Zarif said in a video posted on YouTube that there is "every possibility" of coming to an agreement.
British Prime Minister David Cameron's office said he and Iranian President Hassan Rouhani agreed in a telephone call Tuesday that "significant progress" was made in the last round of talks earlier this month.
The statement said Mr. Cameron also stressed the need for Iran to address the concerns of the international community, which has called for assurances that Iran is not trying to develop nuclear weapons. Iran has repeatedly denied those accusations, saying its nuclear activity is solely for peaceful purposes.
Zarif repeated warnings that Iran will not back down from what it argues is its right to enrich uranium, but said it will no longer insist that Western powers publicly acknowledge that right as a precondition for negotiations.
Also Tuesday, a senior U.S. lawmaker said Congress will not vote on any new economic sanctions against Iran while the talks continue.
Senator Bob Corker spoke after taking part in a meeting with other key Senate leaders at the White House, where President Barack Obama asked for the delay to further pursue diplomacy before considering other steps.
Corker did not offer details of the meeting, and did not say how long a delay the president requested. But he said no new sanctions amendments to the annual congressional defense bill will be considered before the end of the month .