PARIS -- The success or failure of the international initiative to curb Iran's nuclear program hinges largely on an ostensibly clear-cut request: Before talks can begin, Tehran must freeze all activities related to the enrichment of uranium.
But although that demand seems surgically precise, what it means is unclear. It has become the subject of anxious diplomacy around the world - a sort of prenegotiation negotiation - centered on finding a definition that the Iranians and the six countries behind the initiative can accept. The results will determine whether the talks move ahead or fail before they formally start.
Iran has long insisted that it will never give up its "right" to enrich uranium. At the other end of the spectrum, the Bush administration and some nuclear experts have maintained that Iran should not be allowed to spin a single centrifuge - the machines that in large clusters can enrich, or concentrate, uranium into a form that can fuel nuclear reactors or atom bombs.
But in interviews and statements, officials from several countries have begun to show signs of optimism and flexibility, suggesting that players on both sides are struggling to create momentum for talks by finding common ground and avoiding a clash over the issue. The question is whether some low level of enrichment activity, couched as "research," will be deemed permissible and whether the objections to such a move will yield to compromise.
"The definition of enrichment is very important," a senior Iranian official said. "The words in the package are not holy words. This will be a political decision. Each side will have to meet the other halfway."
A formal offer, presented to Iran on June 6, would open talks on an array of incentives and benefits. First, though, Tehran would have to freeze all its enrichment-related activities and commit to keeping them halted during the negotiations.
Diplomats involved in the talks said that Iran's six potential negotiating partners - the United States, Russia, China, France, Britain and Germany - left the definition of suspension vague to maintain unity on their side and to lure the Iranians back to the negotiating table.
"There is no real master plan on what will be suspended," said one European official who, like others interviewed for this article, spoke on the condition of anonymity under normal rules of diplomacy.
By contrast, an accord reached in late 2004 with Iran to freeze its nuclear program "had a long paragraph on the scope of suspension," the official said. "This agreement has a few words." It was after the collapse of that accord that the Iranians restarted their nuclear program.
Russia is trying to serve as mediator and seems to be taking a less rigid view than the Americans and the Europeans to get the talks started, officials said.
On Thursday, President Vladimir V. Putin of Russia, after meeting Iran's president, Mahmoud Ahmadinejad, in Shanghai, said Iran responded positively to the proposals and was ready to talk. But Iranian officials have rejected preconditions for negotiations, and Putin did not say under what circumstances Iran would be willing to talk.
Then, on Friday, Foreign Minister Sergey V. Lavrov of Russia was more explicit, suggesting that Iran had its own preconditions for talks and perhaps its own definition of suspension. "Iran will soon declare the terms for such negotiations," he was quoted as saying by Interfax.
Mohamed ElBaradei, director general of the International Atomic Energy Agency, has floated the idea of a deal in which Iran would halt its industrial enrichment but retain some level of face-saving, low-level work, usually referred to as research. "The big-ticket item is this suspension of the research within Iran," ElBaradei said recently. "We should be able to find a solution to that."
Despite its long-standing objections to Iranian enrichment, the Bush administration recently relented and agreed to leave open in the offer the possibility that Tehran could one day enrich uranium if it proved beyond a reasonable doubt that its program was peaceful. That show of flexibility, diplomats said, might presage a compromise on what Washington would now define as suspension.
The enrichment program starts at Isfahan, where raw uranium is converted into a form that easily becomes a gas. At Natanz, that gas is fed into rows of centrifuges.
Iran is learning how to do such enrichment on an industrial scale and is expected to master the step in the next few years. Although Tehran says its work is solely for peaceful nuclear power, Washington and its allies see the program as a cover for the development of nuclear arms.
Defined narrowly, Iran's suspension of enrichment might consist of nothing more complex than stopping the injection of uranium gas into its centrifuges, while leaving the machines running. (This is standard practice, as slowing the spinning rotors is a risky procedure that can cause them to wobble and crash, destroying the centrifuges.)
Defined broadly, suspension could include halting the spinning machines at Natanz, ending the production of centrifuges and their parts at factories across Iran, stopping the testing of such machines, stopping the import of centrifuge materials and ending the conversion of uranium at Isfahan.
Diplomats said the six nations had agreed to let Iran continue to convert uranium. The question now is where to draw the line on enrichment.
European, Iranian and IAEA officials have said that the technicalities in the incentives package about the enrichment suspension are less important than whether both sides make the political decisions necessary to move forward.