Russia’s foreign minister has said prospects of a preliminary agreement on Iran’s nuclear programme are “very good” on the final day of negotiations.
Sergei Lavrov said he was rejoining the talks in Switzerland on Tuesday, suggesting they were close to a deal.
Marathon negotiations between Iran and foreign ministers from six world powers are nearing a self-imposed deadline.
Ministers want to restrict Iran’s nuclear programme in exchange for relief from crippling sanctions.
There is no sign that the most intractable problems, such as the pace and timing of sanctions relief, have been resolved, the BBC’s Barbara Plett reports from Lausanne.
However, reports say there are plans to issue a joint statement on general points of agreement, accompanied by documents outlining more technical details.
These would be enough for all parties to continue negotiations aimed at achieving a comprehensive accord in June, our correspondent adds.
Mr Lavrov announcement that he was rejoining negotiations followed a statement, as he left the talks on Monday, that he would only return if there was a realistic chance of securing an agreement.
“I believe that the prospects are very good and promising,” he told a news conference on Tuesday.
US Secretary of State John Kerry said talks on Monday had produced “a little more light”.
But he said: “There are still some tricky issues. Everyone knows the meaning of tomorrow.”
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.
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”.
At the scene: Barbara Plett, BBC News, Lausanne
Negotiators worked late into the night and are continuing talks this morning in an all-out effort to meet the deadline.
The six global powers are closer than they have ever been to resolving the longstanding tensions over Iran’s nuclear programme. Progress has been made on steps to curb and monitor Iran’s production of enriched uranium, which can be used to make the core of a nuclear warhead.
But substantive differences remain. These include the pace of sanctions relief and the nature of restrictions on Iran’s nuclear research and development.
If a broad framework agreement is reached by the end of the day, it would be used as the basis of a final accord. No-one here has given a clear answer as to what would happen if it is not.