Iranian nuclear chief Ali Akbar Salehi has said there was agreement on “90 percent of the technical issues" at talks over the country's nuclear program.
Iranian state television quoted Salehi as making the comment on March 17 in Lausanne, Switzerland, where Iranian and U.S. delegations were holding another round of negotiations.
The Iranian official said the sides “have differences only in one major issue which we will try to solve in this evening's meeting" between Iranian Foreign Minister Mohammad Javad Zarif and U.S. Secretary of State John Kerry.
He did not say what that issue was.
The United States and five other powers -- Britain, China, France, Germany, and Russia -- are seeking a deal to rein in Iran's nuclear program, which Western nations fear may be aimed at developing atomic weapons, in exchange for sanctions relief.
The nations have set an end-of-March deadline for a framework agreement, with a full deal to follow by June 30.
U.S. officials sought to temper optimism about an imminent nuclear deal, warning that some difficult issues are yet to be resolved.
"In the mind of [President Barack Obama], the odds have not moved," said White House spokesman Josh Earnest on March 17, warning that "at best" it is a 50-50 proposition.
A senior U.S. official at the talks in Lausanne was quoted as saying on March 17 that the sides “definitely made progress in terms of identifying technical options for each of the major areas."
"There is no way around it, we still have a ways to go," the unidentified official added, indicating there was a significant distance still to be bridged.
Zarif and Kerry met in Lausanne on March 16-17 together with Salehi, U.S. Energy Secretary Ernest Moniz, and others.
The Iranian foreign minister also traveled to Brussels on March 16 to meet with his British, German, and French counterparts, as well as EU foreign policy chief Federica Mogherini.
Mogherini told reporters that the talks had been helpful, saying, "We discussed all the remaining open gaps and the way forward."
She earlier said "there are still some gaps that need to be tackled.”
Kerry said in Egypt on March 15 that "important gaps" remained and Iran must make "important choices" in order to move forward.
He also said that most of the problems holding back an agreement were "political" rather than technical.
He did not elaborate on the issue, but experts say the political matters include the levels of inspections by international monitors and how quickly the international community would scale back the tough economic sanctions against Iran under a deal.
Technical matters expected to be discussed before a deal could be made include how many uranium-enriching centrifuges Iran can keep, the types of centrifuges it can have, and how much plutonium Tehran could produce in a planned heavy-water reactor.
Officials involved in the talks said Western powers were seeking concessions from Iran after the United States and European countries involved in the talks indicated a willingness to compromise on suspending UN sanctions.
The six powers and Iran reportedly started discussing a possible draft UN resolution to endorse any future agreement and to address the lifting of UN sanctions.
Western officials said the UN sanctions could be eased quickly in the event an agreement was reached.
That was seen as a major concession on the part of the United States, which had long insisted that UN sanctions should remain in place for several years after a deal was signed, while U.S. and European sanctions could be removed more swiftly.
Iran wants the UN, EU, and U.S. sanctions lifted at the same time.
Tehran insists that its nuclear program is only for peaceful purposes, but other countries suspect it might harbor covert ambitions to build nuclear weapons.