"We have talked to the Iranians previously via the P5-plus-1 and Mr. Solana about various ways to get to full-blown negotiations," State Department spokesman Sean McCormack said, referring to grouping of Britain, China, France, Germany, Russia and the United States, as well as EU foreign policy chief Javier Solana who has engaged Tehran on nuclear issues.
"In order to get to those full-blown negotiations where you have the US and the other five members of the P5-plus-1 present at the table with the Iranians, they are going to have to suspend their enrichment-related activity.
"And of course there would be a suspension of activities in the (UN) Security Council during that," McCormack added.
Asked about a formula known as "freeze for freeze" – under which Tehran would freeze its nuclear enrichment program at current levels in exchange for no new UN Security Council sanctions – the spokesman did not deny the possibility of implementing such a proposal ahead of further talks.
But he stressed that the six powers were still awaiting a formal response from Tehran to their fresh cooperation offer that Solana presented to Iran in mid-June.
The proposal, aiming at ending the five-year crisis over Tehran's nuclear program, offers technological incentives in exchange for Tehran suspending uranium enrichment.
Tehran denies that its program hides a nuclear weapons quest. US President George W. Bush has said that he has not ruled out using force against Iran to prevent it from acquiring nuclear weapons.
The Islamic republic has put forward its own package, a more all-embracing effort to solve global problems, and notably suggests the setting up of a consortium in Iran for enriching uranium.
A senior State Department official, speaking on condition of anonymity, said that "there have been various discussions about quote-unquote 'pre-negotiations' or how to do a period leading up to full-blown negotiations."
But he stressed that "the conditions for us getting to the table and realizing full-blown negotiations are suspension (of enrichment)."