Adam Ereli, spokesman for the US State Department, said on Monday that Iran must first act like a responsible member of the international community and stop violating its agreements.
"That would represent a sea change in its behaviour," he said. "Then maybe other kinds of notions might be more palatable.
"But right now, I don't think people should be asking the United States, 'Why don't you do this or why don't you do that?'"
Ereli's remarks appeared to dismiss a suggestion by Mohamed ElBaradei, the head of the UN International Atomic Energy Agency.
On Monday while in Stockholm, ElBaradei said that he believed that the US would need to give Iran a security guarantee before a final agreement could be reached on Iran's atomic programmes.
ElBaradei also said the US would need to become more involved in stalled negotiations between Iran and the European Union aimed at persuading Iran to freeze nuclear enrichment permanently.
ElBaradei says the US needs to
be more involved in negotiations
Last week, Robert Joseph, the US undersecretary of state, said that step was the last "red line" Iran needed to cross to produce nuclear weapons.
North Korean talks
In parallel talks aimed at halting North Korea's nuclear weapons programme, the US has offered written guarantees not to attack.
The assurances were offered by Condoleezza Rice, the secretary of state, and her predecessor, Colin Powell.
Unlike the negotiations with Iran, the US is a participant in the North Korean negotiations, along with South Korea, Japan, China and Russia.