"We hope the Iranians will provide a positive answer," to world powers offering incentives for Iran to halt to sensitive nuclear fuel work, national security spokesman Gordon Johndroe said.

But "if they do not, the international community is united that more sanctions are coming," he said.

Earlier Wednesday Iranian President Mahmoud Ahmadinejad vowed that Iran would not yield in the crisis.

"The Iranian people are steadfast and will not step back an inch against the oppressive powers," Ahmadinejad told a rally in the southwestern province of Kohgelouyeh-Boyerahmad.

His comments came after world powers warned Iran has only a fortnight to respond to their latest offer seeking to end a five-year crisis that has raised fears of regional conflict and sent oil prices spiraling.

World powers have offered to start pre-negotiations during which Tehran would add no more uranium-enriching centrifuges and in return face no further sanctions -- the so-called "freeze-for-freeze" approach.

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