Solana said the Iranians were given two weeks to formally respond to the proposal before it would be withdrawn.

At the news conference, Saeed Jalili, Iran’s chief negotiator, refused to answer whether Iran would accept a freeze of its uranium enrichment program, however temporary. But he called the negotiating process a “very beautiful endeavor” with a result that he hoped would eventually be “beautiful to behold.”

Burns did not speak privately with Jalili. But in a brief statement in the morning meeting, he said that the United States was serious in its support for the six-power process and serious that Iran must suspend its production of enriched uranium, the State Department said.

He told his negotiating partners after the talks that the United States would push for new punitive sanctions at the United Nations Security Council in September, one participant in the meeting said.

Saturday’s meeting at Geneva’s City Hall was one of the most important public encounters between an Iranian and an American official since relations were halted after the American Embassy was seized in Tehran in 1979.

Other authorised meetings have occurred. Madeleine K Albright, as secretary of state, for example, once sat at the same table with then Foreign Minister Kamal Kharrazi and other emissaries at the United Nations to discuss Afghanistan. Colin L Powell, as secretary of state, once shook Kharrazi’s hand. American and Iranian officials have met episodically in Baghdad to discuss Iraq’s security.

But Saturday’s meeting was the highest-level session between the countries during the Bush administration, which once branded Iran part of an “axis of evil” and has not ruled out military action against Iran because of its nuclear ambitions.