Three days of talks between Iran and six world powers ended May 16 with no progress toward finding a solution to the standoff over Tehran's disputed nuclear program.
Iran's top negotiator, Deputy Foreign Minister Abbas Araqchi, said that differences were too big to start drafting a final agreement.
Araqchi, however, said the talks would continue and that he hoped a final deal could still be reached by a July 20 deadline.
A senior U.S. official said there needs to be "additional realism," saying that time is not unlimited. He said the talks will resume at an unspecified date next month.
The six world powers -- the United States, China, Russia, Britain, France, and Germany -- want Iran to limit nuclear work in exchange for lifting economic sanctions.
Iran has frozen some nuclear activities for six months in return for minor sanctions relief under a temporary deal that expires July 20.
Based on reporting by Reuters, AP, and AFP