Talks on Iran’s nuclear programme will continue until at least Thursday morning, two days after the original deadline, the US says.
US Secretary of State John Kerry has extended his stay to continue negotiations, officials say.
However, a number of foreign ministers have left the talks and China warned compromise was essential, otherwise “all previous efforts will be wasted”.
A deal would curb the nuclear programme in return for sanctions relief.
Negotiations between the so-called P5+1 – the US, UK, France, China and Russia plus Germany – and Iran continued on Wednesday at Lausanne’s Beau-Rivage Palace hotel after overrunning the self-imposed deadline of 31 March to reach a deal.
On Wednesday evening, a US state department spokeswoman said: “We continue to make progress, but have not reached a political understanding. Therefore, Secretary Kerry will remain in Lausanne until at least Thursday morning to continue the negotiations.”
French Foreign Minister Laurent Fabius is also returning to Lausanne. He had previously said he would return to the talks as soon as it was “useful”.
The P5+1 deal seeks to ensure Iran could not assemble a nuclear weapon in less than a year. The Iranians insist that they have no such ambition.
Earlier, UK Foreign Secretary Philip Hammond told the BBC: “I think we have a broad framework of understanding, but there are still some key issues that have to be worked through.
“Some of them are quite detailed and technical so there is still quite a lot of work to do but we are on it now and we’ll keep going at it.
Mr Hammond stressed again that he would not sign up to a “bad deal”.
Russian Foreign Minister Sergei Lavrov said earlier that “one can say with relative certainty that we at the minister level have reached an agreement in principle on all key aspects of the final settlement of this issue”. He has now left the talks.
Iranian Foreign Minister Mohammad Javad Zarif said that “quite a bit” had been accomplished.
He and Mr Kerry held bilateral talks on Wednesday.
However, Iran’s Deputy Foreign Minister Abbas Araqchi on Wednesday admitted that “problems” remained, saying there could not be a deal without a “framework for the removal of all sanctions”.
The BBC’s Barbara Plett Usher in Lausanne says Mr Araqchi suggested there might be a joint press statement on the progress made and the continuing efforts to try to draft a solution.
This sounds less than the framework on political parameters which the negotiators had been targeting, our correspondent says.
Chinese Foreign Minister Wang Yi, who has now left Lausanne, issued a note of caution.
A Chinese statement read: “It is important to give political guidance to the negotiations… it is important to narrow down the differences.
“If the negotiations are stuck, all previous efforts will be wasted. All parties must be prepared to meet each other half way to reach an agreement.”
Analysis: Barbara Plett Usher, BBC state department correspondent
This is a fluid situation, with little information leaking out of the talks and expectations veering between an imminent deal or none at all.
All negotiators keep saying there has been progress, but not enough yet for an agreement.
The key sticking points are well known: the limits on Iran’s freedom to conduct advanced nuclear research and a framework for lifting UN sanctions. These are more complicated than US and European economic sanctions, because they directly target Iran’s nuclear programme and would be more difficult to re-impose once lifted.
But the main difficulty might be the competing approaches of the two main negotiators, the Americans and the Iranians.
The Obama administration needs as much detail as possible in this preliminary accord to counter opponents in Congress. The Iranians want as little as possible to keep critics quiet while they focus on getting a final comprehensive settlement.
Any agreement would set the stage for further talks aimed at achieving a comprehensive accord by 30 June.
On Wednesday, Israeli PM Benjamin Netanyahu said again that the concessions offered to Iran in Lausanne would ensure a “bad deal” that endangered Israel, the Middle East and the rest of the world.
After months of negotiations, the basic outline of an agreement is well known.
Iran would scale its nuclear programme and subject it to rigorous inspection for at least 10 years. In exchange, there would be an easing and eventual end of crippling UN, US and EU sanctions.
However, there are some issues yet to be resolved. These are thought 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
- Centrifuges – Iran wants to develop advanced centrifuges that can enrich uranium faster and in greater quantities