Nuclear talks between six world powers and Iran could go on past a midnight (22:00 GMT) deadline, US officials say.
Representatives are reportedly preparing to issue a general statement focusing on points of agreement, which would be enough to allow for a new phase of talks on a comprehensive deal.
But there is no sign that the most contentious issues have been resolved.
The world powers want to restrict Iran’s nuclear programme in return for relief from crippling sanctions.
The so-called P5+1 – the US, UK, France, China and Russia plus Germany – are seeking to ensure Iran could not assemble a nuclear weapon in less than a year.
The Iranians insist that they have no such ambition.
Foreign ministers met several times at Lausanne’s Beau-Rivage Palace hotel on Tuesday morning in a high-level push to meet the self-imposed deadline to agree a political framework agreement by the end of the day.
Russia’s top diplomat, Sergei Lavrov, announced that prospects were “very good and promising”, and said he was returning to the talks after leaving for another engagement on Monday.
On Tuesday afternoon, a senior US official said the meetings might go beyond the deadline “if we are continuing to make progress, including into tomorrow, if it’s useful to do so”.
The BBC’s Barbara Plett Usher, who is at the talks, says there is no sign that the most intractable problems have been resolved.
Officials told the Associated Press that there were plans to issue a “framework understanding”, accompanied by documents outlining more technical details.
This would be enough for all parties to continue negotiations in a new phase aimed at achieving a comprehensive accord by 30 June, our correspondent says.
At the scene: Lyse Doucet, BBC News, Lausanne
Six days of intense talks which always ended late and started early are, as predicted, going into the final hours, if not beyond, of a midnight deadline.
The chandeliered rooms at the furthest edge of the Beau-Rivage are packed with journalists and swirling with rumours.
There are a few facts – the Russian Foreign Minister’s return signals progress. There are a few statements – a US spokesman says they are willing to keep working into tomorrow “if useful”. And there are whispered conversations about buses arriving, delegations packing.
Sources say three possible locations have been prepared for the announcement when it comes including two in Lausanne, and one in Geneva at UN headquarters. Iran’s foreign minister, who was ambassador in New York when UN sanctions were imposed, wanted the announcement to be at the UN in Geneva to undo what was done.
But if it goes into the early hours, pragmatism not politics may mean a venue in Lausanne.
Sense of history at Iran talks
After months of negotiations, she adds, the basic outline is well known: Iran would scale back the dimensions of its nuclear programme and subject it to rigorous inspection for at least 10 years. In exchange, there would be an easing and eventual termination of UN, US and EU sanctions.
However, there has not been an agreement on some of the key technical details and political trade-offs involved in making that happen. These are said to include:
- 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 ruled out on Sunday sending its existing stockpile of nuclear fuel abroad, one of the steps demanded by the P5+1.
Israel’s Prime Minister Benjamin Netanyahu is among the opponents of the proposed deal. On Tuesday, he warned it would reduce the time Iran would need to build a nuclear weapon, if it chose to do so, “to under a year and perhaps far less than that”.