The delicate dance between the two sides, along with the other world powers that remain party to the deal, has entered a new phase in recent days after President Joe Biden offered to join direct talks with the remaining participants last week.
"A lot of diplomatic work is underway in order to arrange the meeting to green light the JCPOA and to put all the participants together," Josep Borrell, the European Union's high representative for foreign affairs, said Tuesday, using the acronym for the nuclear deal's formal name, the Joint Comprehensive Plan of Action.
But on Tuesday, Iran halted certain inspections by the International Atomic Energy Agency. Hours later, the U.N. nuclear watchdog issued a report finding the Iranians have amassed a stockpile of uranium enriched up to 20%, according to the Associated Press -- a short technical step away from weapons-grade levels and far beyond what is allowed under the nuclear deal.
Curbing those inspections in particular sparked condemnation from France, Germany and the United Kingdom, the European countries that remain in the agreement. In a joint statement, their foreign ministers said Iran's "dangerous" decision will "significantly constrain the IAEA's access to sites and to safeguards-relevant information."
Still, while Tehran moved ahead with the restrictions, its foreign minister signaled for the first time that the country is open to talks that include U.S. officials.
"We will not have an official meeting because America is not a member of the JCPOA. We are assessing the idea of an unofficial meeting, in which America is invited to as a non-member," Mohammad Javad Zarif said Tuesday.
Such a meeting could come as soon as next month.
The State Department said Tuesday that Iran still has not formally responded to its offer to join talks. On Thursday, State Department spokesperson Ned Price said the Biden administration "would accept an invitation from" Borrell to meet with Iran and the other remaining signatories -- the Europeans, as well as China and Russia.
Former President Donald Trump withdrew the U.S. from the nuclear deal in May 2018 and reimposed U.S. sanctions on Iran -- leaving America at odds with its closest European allies. One year later, Iran followed suit, continually taking steps out of the nuclear deal to pressure the Europeans to make up for U.S. sanctions and provide the economic relief guaranteed under the nuclear deal.
Now, the U.S. is back on the same page with the so-called E3, issuing a joint statement last week that they seek a U.S. and Iranian return to the nuclear deal and new negotiations on extending the pact and addressing Iran's ballistic missiles and regional activity -- something Tehran has said it will not agree to.
"We are working hand-in-glove with our European allies, and we believe that gives us a position of strength when it comes to these negotiations and we believe it provides the best path forward," Price said Tuesday.
Iran's new restrictions on inspections are the latest move in its efforts to raise its leverage.
Zarif announced Tuesday that Iran will no longer share surveillance footage of its nuclear facilities with the IAEA. Instead, the Atomic Energy Organization of Iran said it would retain the footage for three months -- leaving a window to hand it over to the IAEA, but only if there's sanctions relief before then.
IAEA director general Rafael Grossi traveled to Tehran Sunday for last-minute negotiations to stop Iran from implementing the restrictions, which were passed into a law by Iran's parliament in December.
Those talks achieved a "temporary, bilateral, technical understanding," Grossi said Tuesday, that allowed "necessary" monitoring and verification activities to continue for now -- even as Iran has now stopped abiding by its "additional protocol" agreement with the agency, including ending snap inspections of nuclear sites.
"It is our conviction that in doing what we did, we can facilitate a smooth return to the previous situation, if that is possible after the consultations that are going to take place -- and most of all, I think, we facilitated an easier atmosphere and time for the indispensable diplomacy that will be deployed in the next few days, I hope, in order to bring back some stability to a situation that needs it very, very badly," Grossi told the Nuclear Threat Initiative, a Washington-based think tank.
That timeline provides a small "window of opportunity" to put the nuclear deal "back on track," Borrell told the Atlantic Council, a Washington-based think tank. Last Thursday, his deputy invited the U.S. to join the next meeting of the joint commission that meets regularly under the deal's terms, which would be the first U.S. attendance since Trump withdrew in May 2018.
But even if U.S. and Iranian officials are in the room together again soon, it is unclear how both sides will return to compliance -- and whether the U.S. and European push for more negotiations can materialize after U.S. sanctions are lifted.
Iran has so far refused Biden's offer, that the U.S. would return to compliance by lifting sanctions only once Iran met its obligations again, like limiting its stockpile of enriched uranium and the number of centrifuges. Biden's offer has also included a good will gesture to lift severe travel restrictions on Iranian diplomats at the United Nations in New York and rescind a Trump posture that U.N. sanctions on Iran had "snapped back" -- a view that nearly no other country accepted.
Iran will only "follow ACTION w/ action," Zarif tweeted Thursday.
One possible avenue for discreet action may be Iran's oil revenues, frozen by U.S. sanctions in overseas banks. South Korea's Foreign Ministry said Tuesday that Iran had agreed to a proposal to begin unfreezing those funds, but it required U.S. sign-off first.
Price told ABC News, "There has been no transfer of funds," but the U.S. does "discuss these issues broadly with the South Koreans."
That could spark criticism, especially from Republican lawmakers who oppose the nuclear deal and have cast Biden's call for talks as a concession.
"Sanctions should only be relieved for a commensurate change in behavior or policy by Iran," said Behnam Ben Taleblu, calling the potential move "self-defeating" and the possible "beginning of more indirect sanctions relief to Tehran."