"It really needs to be within weeks," Condoleezza Rice, the US secretary of state, told NBC television, referring to a response to a package of perks and penalties from six world powers aimed at halting Iran's enrichment activities.
An Iranian cleric on Friday, however, dismissed Rice's warning of "great costs" if Iran dismissed the offer.
"We are ready to pay a great cost to defend our ideals," Hojatoleslam Ahmad Khatami said in his sermon at the main weekly Muslim prayers in Tehran.
The package, agreed upon on Thursday by the five permanent members of the UN Security Council and Germany, carries the threat of UN sanctions if Tehran remains defiant over what the West calls a rogue nuclear programme that could produce a bomb.
A short statement issued on Thursday night did not mention economic sanctions, but officials said privately that Iran could face tough Security Council sanctions if it refuses to give up uranium enrichment and other disputed nuclear activities, US officials said.
The formal offer of talks is expected to be made by France, Britain and Germany - the three EU nations that previously negotiated with Tehran.
On Friday, the foreign ministers of Germany and Turkey urged Iran to study carefully a proposal aimed at halting work that could produce nuclear weapons, saying it was in Tehran's own interests to avoid confrontation.
Earlier, a senior US state department official said he expected Tehran would be invited to begin new negotiations "within a matter of days".
Russia and China, which both hold vetoes in the Security Council, might also join in any future talks with Iran.
Hojatoleslam Khatami: We are
ready to pay a great cost
In Moscow, Sergey Lavrov, the Russian foreign minister, said on Friday that Tehran that Iran "will not face a deadline to respond to the proposal of the six nations" - but said he expected Iran to give an answer within a few weeks of receiving the offer, the ITAR-Tass news agency said.
The US, in a major policy shift, conditionally agreed this week to join those talks. It would be the first major public negotiations between the two countries in more than 25 years.
Rice suggested in separate comments to National Public Radio that she was ready to meet her Iranian counterpart, Manouchehr Mottaki, if Tehran agrees to suspend activity that can lead to the production of nuclear arms, and to negotiate the details of the deal.