Secretary of State Condoleezza Rice says Iran should give careful consideration to an international package of incentives for halting its uranium enrichment activities.
Rice made the comments in Vienna, where she has been meeting with officials from the five permanent members of the U.N. Security Council, plus Germany.
Secretary Rice says Iran has limited time to respond to the international package, urging a reply within weeks.
But, while stressing the need for a timely response, Rice also seemed to warn against any knee-jerk negative reaction from Tehran. She spoke on the U.S. television network CBS' "Early Show".
"We certainly hope that Iran will take a few days to think over a very serious proposal. It is a proposal that is not just an American proposal, but a proposal that the international community is offering Iran as a way out of this impasse."
Secretary Rice said that the financial and technological incentives being offered have a flip-side: penalties that could be imposed, if Iran rejects the offer.
Iranian officials have cautiously welcomed the prospect of direct U.S. involvement in
stalled European-led talks over Iran's nuclear program, but the government in Tehran has not yet offered an official response to the international proposal.
Speaking on NBC's "Today" program, Rice said Iran has a choice to make.
"Iran now has two paths before it. One is a path that would give Iran considerable benefits, including civil nuclear power. The other, though, is a path that goes to the [U.N.] Security Council again, and can use the full weight of the Security Council
to isolate Iran."
Secretary Rice has said that the impetus for achieving progress in nuclear negotiations now lies with Iran, and the decision it must make. But she denied that the international proposal constitutes an ultimatum to Tehran.
"There is not an ultimatum here. But there is a choice, and the international community needs to know if Iran intends to negotiate seriously."
Iran has long-maintained that its nuclear activities are purely peaceful, with the intent of generating power for a growing population.
Earlier this year, Iran resumed its uranium enrichment program. Depending on the level of enrichment, uranium can be used to generate nuclear power - or to build an atomic weapon.
The Bush administration says it is prepared to join European-led talks with Iran, if Tehran agrees to suspend its enrichment program.
European officials have welcomed direct U.S. involvement in negotiations, but Iran's initial reaction indicated it rejects the U.S. conditions.
Iranian Foreign Minister Manouchehr Mottaki said Thursday his government welcomed dialogue with the United States, but would not give up its rights.