Throughout the 2020 presidential campaign and the transition, Mr. Biden insisted he would lift sanctions imposed by President Donald J. Trump only if Iran returned to the limits on nuclear production that it observed until 2019.
Under the original 2015 deal, Iran shipped 97 percent of its nuclear fuel out of the country and agreed to sharp limits on new production that would essentially assure it would take it a year or more to produce enough material for a single weapon. (It would take even longer to build a weapon.) In return, world powers lifted international sanctions that had choked the Iranian economy.
But over the objections of his first secretary of state and his first defense secretary — both of whom were fired — Mr. Trump restored American sanctions in 2018, arguing that the deal was flawed and that economic penalties would eventually break the government in Tehran, or force it into a new agreement. His move infuriated the other nations that brokered the accord with Iran after years of stop-and-start negotiations.
Over the past year, Iran has unabashedly compiled and enriched nuclear fuel beyond the limits negotiated in the 2015 agreement. Its leaders have accused the United States of being the first to violate its terms and vowed to come back into compliance only after America reversed course and allowed it to sell oil and conduct banking operations around the world.
Publicly, the Iranian foreign minister, Mr. Zarif, has cast doubt that Tehran will agree to talks before the American sanctions are lifted. In a tweet on Thursday, he played down Iran’s repeated violations of the accord as mere “remedial measures.”
A senior Biden administration official said that closing that gap would be a painstaking process.
The offer comes days before a Sunday deadline when Iran has said it will bar international inspectors from visiting undeclared nuclear facilities and conducting unannounced inspections of nuclear sites, unless the United States lifts sanctions reimposed by the Trump administration. The threat would not bar inspectors from declared nuclear-related facilities that are monitored on a regular basis. Still, the ability to inspect anywhere, on demand, by the International Atomic Energy Agency, is mandated by the nuclear deal.
And it is crucial to the international community’s confidence that Iran is not rapidly reconstituting its ability to make a weapon. There has been growing circumstantial evidence, much of it provided by Israeli intelligence, that the country never fully disclosed the sites involved in its program, dating back more than two decades.