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 C.I.A. director, Leon E. Panetta, to the chairman of the Joint Chiefs of Staff, Adm. Mike Mullen, predicted that while the sanctions would hurt Iran, it was unlikely they would prove sufficient to force it to give up the major elements of its nuclear program.

A senior American official said that the United States 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 Ms. Ashton first tell them when sanctions would end, when Israel would give up what it called “the Zionist bomb” and when the United States would eliminate its nuclear weapons.

The new offer would require Iran to send more than 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 American 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 New York Times reported.