European Union foreign policy chief Catherine Ashton said on Wednesday Iran must respond to an offer proposed by world powers if this week's nuclear talks were to succeed, Reuters reported.
Ashton was speaking in the Turkish capital Ankara before talks on the nuclear dispute with Iran in Kazakhstan on Friday and Saturday, at which she will lead negotiations on behalf of the United States, China, Russia, Britain, France and Germany.
"I remain always cautiously optimistic. But I am also very clear that it is very important that we do get a response (from Iran)," she told reporters when asked about the talks in Almaty.
Iran says its nuclear program is peaceful but Western powers suspect Tehran of seeking nuclear weapons capability.
The six powers held talks with Iran in Almaty in February when Western officials say they made a proposal intended to break a deadlock in the dispute over Tehran's nuclear program.
They offered to ease a ban on trade with Iran in gold and other precious metals, and a relaxation of an import embargo on Iranian petrochemical products, the officials say.
In return, Iran would have to suspend uranium enrichment to a fissile concentration of 20 percent at its Fordow facility and "constrain the ability to quickly resume operations there", a U.S. official has said.
The proposals appeared to soften the six powers' previous demands.
Iran has hinted it is not satisfied with the offer, although chief negotiator Saeed Jalili said after the talks in February that the six powers had tried to "get closer to our viewpoint".
Iran says it has a sovereign right to enrich uranium for peaceful purposes, which include fuelling nuclear power plants so it can export more oil. It denies seeking nuclear weapons.