Iran and the European Union's chief negotiators have agreed to resume talks on suspending Iranian uranium enrichment in two weeks time.
But Tehran's representatives ruled out on Saturday in Geneva freezing the Iranian programme in talks that included the United States for the first time.
EU envoy Javier Solana told reporters after discussions with Iranian negotiator Saeed Jalili that the two would speak either by telephone or personally.
"The meeting was constructive, but we didn't get still the answer we were looking for," Solana said.
Jalili said Tehran would remain constructive, but asked that Western powers not turn away from negotiations.
"Iran is calling on the Western powers to resume the dialogue," he added. He ruled out though a freeze on enrichment as a precondition for full negotiations, saying that discussions would only focus on common points of the package.
The US Undersecretary of State, William Burns, who was present, did not speak publicly. Washington's participation had raised expectations of a possible compromise on a formula under which Iran would agree to stop expanding its enrichment activities.
In exchange, six powers - the five permanent United Nations Security
Council members and Germany - would hold off on passing new UN sanctions against the Islamic Republic.
In compliance with the JTI standards