"We accepted the proposal in principle," he said through a translator. "We suggested in the first phase we give you 400 kilograms of 3.5 percent enriched uranium and you give us the equivalent in 20 percent uranium."
Iran has about 3,300 pounds (1,500 kilograms) of low-enriched uranium and needs to refine to 20 percent to operate a research reactor that produces medical isotopes.
The U.S. and its allies fear that if Iran continues to develop its uranium-enriching process, it could eventually develop material for a nuclear weapon, a charge Tehran denies.
The International Atomic Energy Agency proposed in October that Iran ship its uranium out of the country to be further refined by France and Russia and turned into fuel rods, which cannot be turned into weapons.
Iran has been giving mixed signals over the deal, including several statements from lawmakers rejecting it outright.
Mottaki maintained, however, that a clear answer had been given involving the simultaneous exchange of uranium for fuel in stages.
"We gave a clear answer and we responded and our answer was we accepted in principle but there were differences in the mechanism," he said, suggesting the exchange take place on Iran's Kish island, in the Persian Gulf.