Talks to reach a preliminary agreement on Iran’s nuclear programme are entering their final day.
Foreign ministers from six world powers and their Iranian counterpart have been negotiating in Switzerland ahead of a self-imposed deadline.
US Secretary of State John Kerry said talks on Monday had produced “some light” but “tricky issues” remained.
Ministers want to restrict Iran’s nuclear programme in exchange for relief from crippling sanctions.
Iran insists its nuclear programme is for peaceful purposes, but world powers are worried about the country developing nuclear weapons.
They want to keep Iran at least one year away from being able to produce enough fuel for a single weapon.
The final hours of negotiation in Lausanne are taking place between foreign ministers from the so-called P5+1 – comprising the US, UK, France, China, Russia and Germany – and Iranian Foreign Minister Mohammad Javad Zarif. EU foreign policy chief Federica Mogherini is also present.
Mr Kerry said there had been “a little more light” on Monday, “but there are still some tricky issues. Everyone knows the meaning of tomorrow”.
China’s Foreign Minister Wang Yi said on Monday that the “marathon-like” negotiations had entered the final stage and that he was “cautiously optimistic”.
The differences between the parties were narrowing, he said.