LAUSANNE, Switzerland - Iran and major world powers have been making headway in identifying technical options for a historic nuclear deal as an end-March deadline nears but difficult issues must still be addressed, a senior US official said on Tuesday.
Iran and six world powers are seeking an agreement to curb Iran's most sensitive nuclear activities for at least 10 years in exchange for a gradual end to sanctions on Tehran.
The powers - Britain, China, France, Germany, Russia and the United States - aim to complete the framework of a final deal by the end of March and reach a full agreement by June 30.
"We have definitely made progress in terms of identifying technical options for each of the major areas," the US official told reporters on condition of anonymity. "There is no way around it. We still have a ways to go ... But even within this space, we have some tough issues to address."
The official said any framework agreement settled this month would need to have key details, including numbers. "If there is an agreement, I don't see how it could be meaningful without having some quantitative dimensions," he said, without elaborating.
Western and Iranian officials doubted an agreement could be clinched this week and at least one more round of talks would be needed on a deal that could end a 12-year-old nuclear standoff between Tehran and the West over its atomic weapons.
The goal of the negotiations is to arrive at an arrangement whereby Iran would need at least one year to produce enough fissile material -- high enriched uranium or plutonium -- for a single atomic weapon, should Tehran choose to produce one. That is known as the "break-out" time.
The official said the six powers, which have been negotiating with Iran since October 2013, do not share their individual methods of calculating break-out time for Tehran. But they have all reached the same conclusions, he noted.
The official offered no details on the options under discussion. But negotiators say they are looking at a maximum number of enrichment centrifuges Iran could retain, the size of its future uranium stockpiles and other limitations Tehran would be subject to for at least 10 years.
The head of Iran's Atomic Energy Organisation, Ali Akbar Salehi, was more upbeat after meetings with US Energy Secretary Earnest Moniz in the Swiss city of Lausanne, where negotiations are taking place.
"We have made progress on technical issues," Salehi told reporters. "One or two issues remain and need to be discussed."