As a possible compromise, experts say, Iran might agree to refrain from starting up the plant for the six months of an interim agreement, while continuing some work on the installation.

Iran has made clear it has no intention of suspending the enrichment of low-enriched uranium, either under an interim agreement or as part of a comprehensive accord.

But American officials still want to curb Iran’s ability to make a bomb in a matter of months. This could be done by banning Iran from enriching uranium to 20 percent and arranging for Iran to convert its current stock of such uranium into oxide form, which is harder to convert to weapons grade. Such a step, many experts say, would be more effective if it was also coupled with constraints on the number and type of centrifuges Iran is permitted to retain.

“If there aren’t more constraints put on the Iranian centrifuge program, then you haven’t accomplished very much at all,” David Albright, the president of the Washington-based Institute for Science and International Security, said on a recent conference call organized by the Israel Project, a nonprofit organization.

American officials are believed to have sought a commitment by Iran that it would not operate sophisticated centrifuges, known as IR-2s, or install new ones. Banning Iran from manufacturing additional centrifuges of all types would also slow progress toward a bomb, but Mr. Albright said it might not be part of the Obama administration’s proposal.

To induce Iran to halt its program, the United States is proposing freeing up billions of dollars in Iranian funds that have been frozen in banks overseas, and could be given to Iran in installments in return for concessions. But on Friday, an Iranian negotiator said Iran expected relief from sweeping sanctions against its oil and banking industries. “We have announced to the West that in the first phase the issue of banking and oil sanctions must be considered,” said the representative, Majid Takht-Ravanchi, according to the Iranian news agency Mehr.