Condoleezza Rice said the United States was prepared to join European Union powers Germany, France and Britain to meet Iran if it abandoned this part of its nuclear programme.
Iran, which is the world's fourth largest oil exporter, says its nuclear programme is for civilian energy-production. Washington and its allies accuse it of seeking to build a bomb.
Rice said: "To underscore our commitment to a diplomatic solution and to enhance prospects for success, as soon as Iran fully and verifiably suspends its enrichment and reprocessing activities, the United States will come to the table with our EU3 colleagues and meet with Iran's representatives."
Rice leaves for Vienna later on Wednesday to meet the permanent five members of the UN Security Council and Germany to discuss a package of incentives and threats to get Iran to abandon its nuclear prgramme.
Rice said the US had agreed on the "essential elements" of the package and she hoped Iran's government would thoroughly consider the proposal.
A senior state department official said Russia and China also supported the package but final details still had to be worked out.
"The negative choice is for the regime to maintain its current course, pursuing nuclear weapons in defiance of the international community and international obligations," said Rice in excerpts of remarks to be delivered later on Wednesday.
"If the regime does so, it will incur only great costs," she added.