WASHINGTON — The Obama administration and its European allies are preparing a new offer for negotiations with Iran on its nuclear program, senior administration officials say, but the conditions on Tehran would be even more onerous than a deal that the country’s supreme leader, Ayatollah Ali Khamenei, rejected last year.

Iran’s reaction, officials say, will be the first test of whether a new and surprisingly broad set of economic sanctions is changing Iran’s nuclear calculus. As recently as last summer, senior officials, ranging from the CIA director, Leon Panetta, to the chairman of the Joint Chiefs of Staff, Adm. Mike Mullen, predicted that while the sanctions would hurt Iran, they would not be sufficient to force it to give up the major elements of its nuclear program.

A senior U.S. official said on Wednesday that the U.S. and its partners were “very close to having an agreement” on a common position to present to Iran. But the Iranians have not responded to a request from Catherine Ashton, the European Union’s foreign policy chief, to meet in Vienna in mid-November. Iran insisted that Ashton first tell them when sanctions would end, when Israel would give up what it called “the Zionist bomb” and when the U.S. would eliminate its nuclear weapons.

The new offer would require Iran to send roughly 4,400 pounds of low-enriched uranium out of the country, an increase of more than two-thirds from the amount required under a tentative deal struck in Vienna a year ago. The increase reflects the fact that Iran has steadily produced more uranium over the past year, and the U.S. goal is to make sure that Iran has less than one bomb’s worth of uranium on hand.

Iran would also have to halt all production of nuclear fuel that it is currently enriching to 20 percent — an important step on the way to bomb-grade levels. It would also have to make good on its agreement to negotiate on the future of its nuclear program. The failed 2009 accord was scuttled by hardliners in Tehran. A later analysis by intelligence analysts concluded that Khamenei personally rejected the deal, reversing the judgment of President Mahmoud Ahmadinejad.

For that reason, many officials suspect that this latest initiative is likely to fail. But they say that it fulfills President Barack Obama’s promise to keep negotiating even while the pressure of sanctions increases.

“This will be a first sounding about whether the Iranians still think they can tough it out or are ready to negotiate,” one senior U.S. official said this week, declining to be identified because Washington and its European allies are still debating the final details of the package they will present to Iran. “We have to convince them that life will get worse, not better, if they don’t begin to move.”

While Ahmadinejad said in New York last month that Iran was ready to return to negotiations, the country has not yet set a date to meet. Khamenei, speaking earlier this week in Qom — not far from the underground enrichment plant whose existence was exposed last year — did not sound in a mood to compromise.

“The world bullying powers have created brouhaha about sanctions on Iran,” he was quoted in the Iranian press on Tuesday as saying to clerics and students, “but this nation has overcome sanctions over the past 30 years with its patience and resistance.” While all sides have expressed a willingness to engage in new negotiations, what is happening now is more brinksmanship than give-and-take. U.S. officials say it is not crucial for Iran to return to the negotiating table in earnest right away; the longer they wait, the more time available for sanctions to bite.

Already Iran has faced difficulties refueling airplanes in Europe, getting some ports to accept their ships and attracting much-needed investment for oil production, officials and analysts say.

But the longer the stalemate lasts, the more uranium Iran produces. According to international nuclear inspectors, Iran already has enough fuel for two bombs, though U.S. officials estimate it would take at least a year to enrich its current stockpile to bomb-grade and then fabricate it into a weapon. “That’s a long time,” one official said recently, enough for the U.S. or Israel to take military action to stop the program, he contended.

In public statements recently, the administration has tried to harden its rhetoric about the dangers of an Iranian weapon.

Gary Samore, Obama’s coordinator for countering weapons of mass destruction, told an audience at the American Association for the Advancement of Science in Washington last week that if Iran acquired a weapon, it “would have an utterly catastrophic effect” in the region. If successful, Iran could drive other states in the Persian Gulf to seek their own nuclear weapons. An attack by Israel on Iran’s facilities, he added, could set off a regional war. Samore described stopping Iran’s program as his “No. 1 job.”

Two years into office, Obama has organized an impressive sanctions regime and managed to combine diplomacy and pressure better than many experts had predicted. But so far he has little to show for it, which has prompted a discussion inside the White House about whether it would be helpful, or counterproductive, to have him talk more openly about military options.

Several European officials have discouraged that approach. But they also worry that negotiating about the fate of uranium that Iran has enriched in violation of U.N. Security Council commands could have the effect of convincing the Iranians they could retain some of their enrichment capability at the end of any negotiation.

Obama’s campaign in 2008 said that would be unacceptable; as president, he has not addressed the question clearly.

© 2010 The New York Times Company