Updated

WASHINGTON -- Secretary of State Hillary Clinton said the United States has a "very serious" disagreement with Brazil over its approach to curbing Iran's nuclear program.

In remarks Thursday at the Brookings Institution think tank, Clinton said a nuclear fuel swap proposed jointly by Brazil and Turkey would only serve to buy time for Iran to pursue its nuclear ambitions. And she said it would undercut the international community's ability to present a united front against Iran.

Clinton said the U.S. view -- not shared by Brazil -- is that Iran will agree to negotiate over the nuclear program only after the United Nations Security Council imposes tough new sanctions.