EU foreign policy chief Javier Solana says his key nuclear talks with Iranian envoy Ali Larijani will now take place on Saturday, but he announced no venue.
Iran says it has a right to a nuclear industry
Talks had been tentatively set for Wednesday in Vienna but Iranian envoys said they had been postponed for several days.
The talks follow Iran's refusal to meet a 31 August UN deadline to stop uranium enrichment, sparking sanctions calls.
Six world powers have offered Iran a range of incentives to stop enrichment.
Mr Solana told reporters in Denmark: "I have said it will be in two days. Today is Thursday so it will be Saturday. I prefer not to say the place."
Iran says its nuclear programme is purely peaceful but the US and other Western nations fear Tehran may make nuclear weapons.
The five UN Security Council members plus Germany, who offered the incentives package to Iran, are meeting in Berlin on Thursday to discuss a strategy on the issue.
The US has said a UN vote on sanctions could come this month.
Russia and China have previously opposed sanctions, but Russian Foreign Minister Sergei Lavrov indicated on Wednesday that Moscow was considering the possibility.
However, BBC diplomatic correspondent Jonathan Marcus says both the Russians and the Chinese are still stressing the importance of negotiations.