Supreme leader Ayatollah Ali Khamenei said on Tuesday that Iran will reject any dialogue if its outcome is pre-determined by Washington, raising the possibility a mooted nuclear fuel deal may be derailed. Khamenei's comments came as world powers turned up the heat on Iran to accept the UN-brokered nuclear deal, with US Secretary of State Hillary Clinton saying Iran was at a "pivotal moment" to show it did not want to be isolated from the international community.

Iran is engaged in high-profile UN-hosted talks with world powers over how to procure nuclear fuel for a Tehran research reactor.

But officials here are increasingly opposed to the proposed deal under which Iran would send low-enriched uranium (LEU) abroad for conversion into fuel for the Tehran reactor. They say they would rather buy the fuel directly. Today, Khamenei, Iran's all-powerful leader who has the final say in all national issues, raised the possibility the UN-brokered deal may fail.

"We do not want any negotiation, the result of which is pre-determined by the United States," Khamenei said in a speech on the eve of Wednesday's 30th anniversary of the US embassy seizure by radical students.