US Energy Secretary Ernest Moniz was headed to Geneva Saturday to join talks with Iranian officials as efforts for an historic nuclear deal stepped up a gear.
It was the first time that Moniz, who is himself a nuclear physicist, had joined the negotiations, although US energy officials have been involved in the intensive technical talks under way ahead of new meetings between the countries' top diplomats on Sunday and Monday.
"At the request of Secretary of State John Kerry, Secretary of Energy Ernest Moniz will travel to Geneva on Saturday to join Secretary Kerry in continued negotiations with Iran over its nuclear programme," an energy department spokesman said.
"Department of Energy officials have consistently been involved in these in-depth technical discussions as part of the US negotiating team and Secretary Moniz will be joining the team in Geneva to continue these ongoing detailed technical deliberations."
A second day of technical talks was to be held on Saturday and the director of the Iranian Atomic Energy Organisation Ali Akbar Salehi flew to Geneva to meet Moniz as part those discussions, Iranian officials said.
"Salehi will hold talks on the technical aspects of the negotiations with the P5+1 group with US Energy Secretary Ernest Moniz," IAEO spokesman Behrouz Kamalvandi said.
Senior negotiators from the six powers -- Britain, China, France, Germany and Russia as well as the United States -- are also to meet in Geneva on Sunday in a bid to drive the talks forward, the European Union said.
Zarif and Kerry have held repeated meetings in recent weeks as the clock ticks down to March 31 for the political framework of a comprehensive deal reining in Iran's nuclear programme in return for the lifting of Western sanctions.
That target aims to ensure that a full agreement including all the technical details can be signed by a June 30 deadline, which looms all the larger after two previous deadlines were missed.
But Iranian officials have since voiced unhappiness with separating the political and technical aspects of an agreement.
"We won't have a two-stage deal," deputy foreign minister Abbas Araghchi said.
"After a year of negotiations, we must tackle the details and all the more so as we want to have both the general framework and the details in the final agreement."
The technical aspects include the detailed arrangements for Iran's future enrichment of uranium.
The process produces fuel for nuclear reactors but in highly extended form can also produce the core of an atomic bomb, the source of international concerns about Iran's ambitions.