Iran agreed on Monday to ship much of its low enriched uranium abroad in a nuclear fuel swap deal backed by Turkey and Brazil, but the United States said moves for toughened sanctions would still go ahead.
The accord, which commits Iran to depositing 1,200 kilograms (2,640 pounds) of low enriched uranium (LEU) in Turkey in return for fuel for a research reactor, was signed by the foreign ministers of Iran, Turkey and Brazil.
Tehran, already under three sets of UN sanctions over its defiant nuclear drive, touted the agreement as a goodwill gesture that paves the way for a resumption of talks with world powers.
But the United States said it would not halt or slow its drive for toughened sanctions against the Islamic republic.
"It does not change the steps that we are taking to hold Iran responsible for its obligations, including sanctions," White House spokesman Robert Gibbs said.
Gibbs, who had earlier issued a written statement expressing "serious concerns" about the deal, said that if Tehran lived up to the new pact it would represent "some progress."
But even if that took place, Gibbs said Washington had concerns about the "overall thrust" of the nuclear programme and the fact Tehran said it would continue enriching uranium to 20 percent.
State Department spokesman Philip Crowley said: "There are those who might characterize this as a breakthrough. I think we remain skeptical that this represents anything fundamentally new."
However, he said US officials were still evaluating the accord, and planned to consult with its international partners in the coming days, including with Brazil and Turkey.