THE OBAMA ADMINISTRATION says it expects Iran’s new leadership to show its seriousness about striking a deal on its nuclear program by offering a response in Geneva this week to a proposal that the United States and its partners put forward this year. That confidence-building plan calls for Iran to freeze its higher-level enrichment of uranium and accept more inspections in exchange for the easing of several second-order sanctions, including a ban on trading in gold. Several reports, including one in the Wall Street Journal, have said that Tehran may be preparing an offer that meets several of those terms, including the enrichment restriction.

If so, that would signal a major change in position by supreme leader Ayatollah Ali Khamenei, who has repeatedly rejected both interim and comprehensive offers to end the standoff over the nuclear program. But the Obama administration should not necessarily be prepared to accept an Iranian “yes” for an answer, even if it is unqualified. That is because Iran’s continued development of its nuclear infrastructure during the course of this year has torn some big holes in what was intended to be a temporary safety net.

A year ago, Iran’s growing stockpile of uranium enriched to 20 percent appeared to be the most dangerous piece of its nuclear infrastructure, because that material could be quickly converted to bomb-grade. The enrichment takes place in an underground facility that has little plausible use other than for weapons production. A freeze or shutdown of that plant and the securing of the material already produced, if accepted by Tehran even six months ago, would have eased the threat that Iran could race to produce a bomb sometime soon.

Since then, however, Iran has begun installing a new generation of centrifuges at its largest enrichment plant, in Natanz. Because they can process uranium far more quickly, these new machines create a threat of an Iranian nuclear breakout beyond that posed by the 20 percent stockpile. Meanwhile, a new reactor based on heavy-water technology, in Arak, is due for completion next year and would allow Iran to produce plutonium that could be used in bombs.

Any accord with Iran, even an interim arrangement, must take these new facts into account. No sanctions relief should be granted unless Iran takes steps that decisively push back its potential time frame for producing the core of a nuclear warhead. That means that the advanced centrifuges and the Arak reactor must now be part of any deal.

Even such a broadened interim arrangement would draw strong objections from Israel, which says that no sanctions should be eased unless Iran gives up all uranium enrichment. Israeli officials argue that even a slight easing of sanctions would cause the system to quickly crumble, as Russia, China, Turkey, India and other nations that have only reluctantly honored the sanctions rush to resume normal trade. Iran might then win sufficient relief to succor its economy while retaining a robust nuclear capacity.

All the difficulties with a preliminary accord might be addressed if it were folded into a larger framework that spells out how much of its nuclear program Iran is prepared to give up in exchange for full sanctions relief. That, in turn, invites the ultimate test of the new government: Is it willing to definitely abandon its drive for a nuclear weapon or only temporize?