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.
Correspondents say difficulties remain, despite statements from officials saying definite progress had been made.
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”.
The differences between the parties were narrowing, he said.
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.
Sense of history at Iran talks
Ministers are aiming to agree on a political framework agreement by Tuesday night that would lead to a final and comprehensive accord by 30 June.
Senior Iranian negotiator Abbas Araqchi told Iranian TV that he was “hopeful” about a deal, but that ministers were not in a position to say whether they were close to resolving all the issues.
Iranian and Western officials have said that a deal is possible, but after almost 18 months of negotiations several sticking points remain.
Three of the major outstanding issues are:
- Length of restrictions – Iran’s nuclear activities would be strictly limited for at least 10 years. After that, Iran wants all limits to be lifted. The P5+1 says they should be removed progressively over the following five years
- Sanctions relief – Iran wants the UN sanctions suspended soon after an agreement. The P5+1 says they should be eased in a phased manner, with restrictions on imports of nuclear-related technology remaining for years
- Non-compliance – The US and its European allies want a mechanism that would allow suspended UN sanctions to be put back into effect rapidly if Iran reneges on a deal. Russia reportedly accepts this, but wants to ensure its Security Council veto rights are protected
Another point of contention is Iran’s desire to be able to develop advanced centrifuges, which could enrich uranium faster and in greater quantities. While enriched uranium is used as fuel for nuclear reactors, it can also be used to make nuclear bombs.
Adding to the list of issues to be resolved, Iran’s lead negotiator has ruled out sending its existing stockpile of nuclear fuel abroad, one of the steps demanded by the P5+1.
Meanwhile, Israel’s Prime Minister Benjamin Netanyahu reiterated his opposition to a deal, saying it would send the message “that Iran stands to gain by its aggression”.