"This is a pivotal moment for Iran. We urge Iran to accept the agreement as proposed and we will not alter it and we will not wait forever," she told reporters in Washington.
Tehran and Washington have been at odds for years over Iran's nuclear program, which Western powers fear is a covert effort to develop nuclear weapons. Iran has denied that and says it needs nuclear technology to generate electricity.
Western powers want Iran to accept the draft deal under which it would send most of its low-enriched uranium (LEU) abroad by the end of the year for further enrichment to turn it into fuel for a reactor in Tehran.
But on Monday an Iranian diplomat said that additional talks were needed and that Tehran wants to import atomic fuel rather than send its own uranium abroad for processing.
The plan drafted by the U.N. nuclear watchdog calls for Iran to transfer the LEU to Russia and France to produce fuel for a Tehran reactor that makes isotopes for cancer treatment.
Russia, France and the United States, which would help modernize the reactor's safety equipment and instrumentation under the plan, see it as a way to reduce Iran's LEU stockpile below the threshold needed to get material for a bomb.
But since the October 19-21 talks, Iran has made clear it is loath to ship its own LEU abroad because of its strategic value, and would prefer buying the reactor fuel from foreign suppliers.
Western diplomats say the three powers do not want more talks and that Iran's demands would do nothing to remove the risk of nuclear proliferation in Iran.
"We are speaking with one voice on this critical issue," Clinton said, referring to the five permanent members of the U.N. Security Council and Germany.
(Reporting by Paul Eckert; Editing by Xavier Briand)
