Iran and the United States Tuesday returned to the negotiating table for a second day of talks, as a political storm over the issue unfolded thousands of miles away in Washington.
By the shores of Lake Geneva in the town of Montreux, US Secretary of State John Kerry met with his Iranian counterpart Mohammad Javad Zarif as they try to pin down a political framework for a deal to rein in Tehran's nuclear programme by a March 31 deadline.
US officials said they began their talks at 9:33 am (0833 GMT).
After months of discussions, the two men launched this latest round of talks on Monday, and are due to continue negotiating until Wednesday afternoon, when Kerry will fly to Riyadh to meet King Salman.
Few details of the emerging deal have publicly come to light so far, but aides to Israeli Prime Minister Benjamin Netanyahu have threatened that may change when the Israeli leader makes a controversial address to the US Congress later Tuesday.
Kerry and his staff have warned Netanyahu against betraying US trust by revealing classified briefings about the course of the negotiations.
Netanyahu's lobbying trip to Washington is seen as a last-ditch bid to derail one of the last key goals of President Barack Obama's foreign policy.
But the US insists that a deal forged through diplomacy would be the best way to ensure Iran does not acquire a nuclear bomb.