Talks to resurrect negotiations for a return to the 2015 Iran nuclear deal looked less likely to proceed after EU and Iranian officials contradicted each other.
Iran had said its lead nuclear negotiator would be in Brussels within days for talks with the EU.
But Josep Borrell, the bloc’s foreign affairs chief, declined to confirm reports that the meeting would take place on Thursday.
A “critical point in time” had been reached with regards to the deal, he said.
An EU spokeswoman had earlier cast doubt on the status of the talks.
On Monday, France’s foreign ministry spokeswoman, Anne-Claire Legendre, said Paris had not been informed of any meeting in Brussels later this week and that any such meeting could not replace the talks in Vienna.
A senior EU official was in Tehran last week amid efforts to restart discussions.
Indirect, EU-convened negotiations in Vienna between Iran and the US over the resurrection of the accord have been on hold since June, when Iranian presidential elections were held.
Mr Borrell said all parties were “working hard to go back to Vienna” but told the Iranians “time is not on their side”.
The US and EU are seeking a return to the Vienna talks, which were also attended by the remaining signatories to the deal: Britain, China, France, Germany and Russia.
Iranian foreign ministry spokesman Saeed Khatibzadeh said that “after the talks in Tehran, [lead nuclear negotiator Ali] Bagheri will go to Brussels to discuss outstanding issues”.
Mr Khatibzadeh said “obstacles and challenges behind the lack of progress in six rounds of talks in Vienna must be discussed” with the EU in Brussels.
He referred only to the EU, and not to the other signatories to the deal.
On Monday, Mr Borrell said he was “more optimistic today than yesterday” that the talks with Iran would be held this week.
“No confirmation yet, but things are getting better. And I hope that we will have preparatory meetings in Brussels in the days to come. But I cannot be sure on that,” he said.
Enrique Mora, the EU official who convened the six rounds of Vienna talks, visited Iran last week. But no date was agreed on when those negotiations would resume.
The 2015 deal gave Iran sanctions relief in return for limits on its nuclear programme, but has in effect been void since former US president Donald Trump abandoned it in 2018 and reimposed heavy economic measures.
Iran says it wants the negotiations to resume but that Washington must first lift its sanctions.
“Our main goal is to make sure that all the sanctions will be lifted and obtain guarantees so that nobody in Washington can ever again play with the world and ridicule it [by breaching the deal]. This goal can only be achieved through pragmatic talks,” Mr Khatibzadeh said.