In remarks to members of the European parliament, Bildt spoke of "difficult choices and judgements that we have to do in the next, not only weeks, but months, but not very much longer than that."
His comments came after US Secretary of State Hillary Clinton said that Washington was committed to talks with Iran "but the time for action is now. The opportunity will not remain open indefinitely."
International talks on curbing the Islamic republic's nuclear ambitions have ground to a halt since the contested June 12 presidential elections, which sparked riots leading to the deaths of at least 20 people and mass arrests.
Major world powers, led by EU nations, have been trying to convince Iran to halt uranium enrichment in exchange for political and economic incentives.
Enrichment is a process for powering a nuclear reactor, but at highly refined levels the uranium can be used to build the core of an atom bomb, which many countries fear the Islamic Republic is trying to covertly develop.
Iran, which is labouring under three sets of UN sanctions, insists it is only developing a civilian programme and has refused to sit down at the negotiating table if it has to suspend enrichment before the talks begin.
Bildt said he had seen press reports that Tehran might be ready to respond soon.
"We look forward to looking at that," he said, but added: "Whether that will be the beginning of a constructive process on this particular issue, that is too early to judge."
He insisted that dialogue was the only solution.
"There may be others who believe there are far more simple solutions to that. I don't think there's any solutions if we don't engage in a true diplomatic dialogue."
In Tehran on July 11, Iranian Foreign Minister Manouchehr Mottaki told reporters: "we are going to present our package which will be a basis to negotiate all regional and international issues."
He did not elaborate, and no official contact has been made with the EU.
