With the clock ticking, Kerry and Iranian Foreign Minister Mohammad Javad Zarif and their teams huddled today in the Swiss resort town of Lausanne on Lake Geneva trying to overcome still significant gaps after nearly two years of negotiations between Iran and the five permanent members of the UN Security Council and Germany.
The top diplomats from Britain, China, France, Germany and Russia are expected to join the talks if the US and Iran are close to an agreement.
The official was not authorised to discuss the talks by name and spoke on condition of anonymity.
The pressure is high. The seven nations have set themselves a March 31 deadline for the outline of a final accord they hope to seal by the end of June. Both President Barack Obama and Iran's Ayatollah Ali Khamenei have spoken against what would be a third extension of the talks.
At the opening session of today's talks neither Kerry nor Zarif responded to reporters' questions about whether the situation in Yemen would be discussed.
Opponents of a nuclear deal, among them wary American allies in the Middle East and hardliners in Iran and in Congress, stand ready to complicate the process if negotiators cannot reach a breakthrough in the next six days.
The United States and its partners are trying to get Iran to cut the number of centrifuges it uses to enrich uranium, material that can be used in warheads, and agree to other restrictions on what the Islamic Republic insists is a peaceful nuclear program.
Speaking yesterday morning to US ambassadors in Washington, Kerry assailed opponents of a deal.
