Western officials told Iran on Sunday that negotiations to revive the 2015 nuclear deal with six world powers cannot continue indefinitely, as the sides announced a break after presidential elections in Iran.
Negotiations have been taking place in Vienna since April to work out how Iran and the US can return to compliance with the nuclear pact, which Washington abandoned in 2018 under president Donald Trump and reimposed sanctions.
Iran responded by increasingly breaching the terms of the agreement.
The pause in the talks came after Ebrahim Raisi, a hardliner and fierce critic of the West, won Iran's presidential election on Friday. Two diplomats said they expected a break of about 10 days.
Mr Raisi will take office in August, replacing pragmatist Hassan Rouhani, under whom Tehran struck the deal agreeing to limit its nuclear programme in return for the lifting of international sanctions.
Iranian and western officials say Mr Raisi's election is unlikely to alter Iran's negotiating position.
Iran's hardline supreme leader Ayatollah Ali Khamenei has the last word on all major policy.
But some Iranian officials suggested that Tehran could have an interest in pushing through an agreement before August to give Mr Raisi a clean slate.
An Iranian government official told Reuters that if a deal were finalised before Mr Raisi took office, he would be able to deflect blame for any concessions on to his predecessor.
"Rouhani, not Raisi, will be blamed for any future problems regarding the deal," the official said.
Britain, France and Germany have been acting as mediators, shifting between the Iranian delegation and a US team that is not a formal participant.
The western countries said the longer Iran breaches the deal and produces banned nuclear material, the harder it becomes to restore the pact.
"As we have stated before, time is on nobody’s side. These talks cannot be open ended," the European diplomats said.
They said the most difficult issues are yet to be resolved.
US National Security Adviser Jake Sullivan told ABC News that there was still "a fair distance to travel", including on sanctions and the nuclear commitments Iran has to make.
With the talks on pause, attention will now turn to extending a separate accord between Iran and the UN nuclear watchdog IAEA, which expires on June 24.
Iran has ended extra monitoring measures that were introduced under the 2015 deal.
EU political director Enrique Mora, who is co-ordinating the nuclear talks, said he expected an extension that would let data continue to be collected while placing limits on the IAEA's access to it for now.
While a hardliner was expected to succeed Mr Rouhani, it could play into the hands of the deal's opponents on the right in the US, and in Israel and Arab countries, who say Iran is not reforming and not trustworthy.
On Sunday, Israel's new Prime Minister Naftali Bennett said a Raisi government would be a "regime of brutal hangmen" with whom world powers should not negotiate a new nuclear accord.
Mr Raisi is under US sanctions over a past that includes what the US and human rights groups say was the extrajudicial killing of thousands of political prisoners in 1988.
He has never publicly addressed such allegations.
Mr Raisi, like Mr Khamenei, has supported the nuclear talks as a way to cancel US sanctions that have damaged Iran's economy.
Several Iranian officials said the negotiating team would remain intact for the next few months.
"Who Raisi picks as his foreign minister will reveal the new government's foreign policy approach," another official said.
The official said that nuclear policy was decided by Mr Khamenei.