U.S. negotiators return to Vienna late this week for more talks with Iran over a return to the 2015 nuclear deal, unsure of whether they are on the brink of agreement or headed toward a prolonged, and possibly unsuccessful, slog of extended negotiations.

The indirect talks, now entering their fourth round, “have been serious, the mood has been constructive,” a senior State Department official said Thursday. “We have to see whether the next round actually moves forward” or Iran continues to make what the official called “unrealistic demands.”

“We think that it’s doable, because it’s not rocket science. It’s not a new deal,” said the official, who briefed reporters on the condition of anonymity under rules set by the State Department. “It’s reviving one that has been undermined over the last several years.”

“If Iran makes that political determination that it is not going to ask for more” than U.S. compliance with the deal requires, “and it’s not going to do less in terms of its nuclear commitments . . . then it can be done relatively quickly.”

But the two sides, on Iranian insistence talking indirectly through the other signatories to the agreement — Britain, France, Germany, Russia and China — still disagree on what compliance means, and what it would take to achieve it.

President Donald Trump withdrew from the accord three years ago. Since then, both the United States and Iran have taken steps far from its original terms, under which U.S. and international sanctions were lifted and Iran agreed to strict limits and monitoring of its nuclear program.

Among the thorniest issues is what to do about the many advanced centrifuges — which can more quickly enrich uranium into fissile material — Iran has begun to operate in response to Trump’s “maximum pressure” sanctions designed to cripple its economy.

Iranian news media, citing government sources, said Thursday that the United States and its European partners have demanded the centrifuges be “destroyed.” Iran maintains that under the agreement, they must only be disabled and stored under the supervision of the International Atomic Energy Agency.

A U.S. official familiar with the negotiations declined to characterize the specifics of the U.S. position, but said there is a “clear statement [in the agreement] of what their nuclear program should be like” at this point in time, “and they are in excess of this. They are operating centrifuges they are not supposed to be operating, in greater number. That needs to be reversed.”

The agreement, known as the Joint Comprehensive Plan of Action, or JCPOA, includes sharp restrictions on the number and type of advanced centrifuges Iran can develop and operate for research purposes, as well as a prohibition on using them to produce enriched uranium until at least 2026.

U.S. and European officials agree that the knowledge Iran has gained on how to produce and operate the advanced centrifuges cannot be eliminated. But they would like to increase the amount of time that the Iranians would have to put that knowledge to use by getting rid of the machines, rather than storing them.

To some extent, both sides can argue that their interpretation of the deal is the correct one.

“The JCPOA’s negotiators did not anticipate that President Trump would unilaterally withdraw from the agreement, reimpose sanctions and essentially invite Iran to exceed the advanced centrifuge limits without the approval of the Joint Commission,” said Daryl G. Kimball, executive director of the nongovernmental Arms Control Association, referring to the body of representatives from all countries in the deal that rules on its implementation.

“It is not surprising the two sides disagree on what it will take for Iran to return to compliance within the terms of the original agreement,” Kimball said. “From a nonproliferation standpoint, it may be ideal for the excess advanced centrifuges to be verifiably destroyed. . . . But it would not be inconsistent with the JCPOA if the two sides were to agree” to disable the machines “and put them in cold storage and under IAEA seal,” he said.

Putting the centrifuges under IAEA supervision became less attractive when Iran moved early this year to limit the agency’s visibility into its nuclear program and threatened to shut down the intrusive monitoring under the deal altogether. Although Iran and the IAEA agreed to a temporary fix, that deal expires in two weeks unless it is extended, or the larger JCPOA negotiations succeed.

U.S. negotiators believe they have been more than forthcoming on the sanctions they are prepared to lift — although Iran disagrees — and that the ball is now more than ever in Iran’s court to make “the political decision that it genuinely wants to return to the JCPOA,” the State Department official said.

“But we don’t know. . . if Iran has made that decision. We don’t know if they’ve decided that they’re prepared for a strict mutual return to compliance, and whether they’re prepared to do so now.”

As they resume the talks, U.S. officials were additionally aggravated by Iranian reports last weekend that a deal for the release of four Iranian American detainees imprisoned by Iran was in the works.

Calling it the “height of cruelty” to use wrongfully detained U.S. citizens “as pawns to try to extract concessions,” the State Department official said that Iran had “exceeded that cruelty. . . by leaking information that a deal had been reached” for their release.

“We would really hope that Iran would not subject the detainees and their families to what they’ve had to endure. . . when, for a brief moment, they read the news that their loved ones were coming home because a deal had been struck.”

While the United States is engaged in ongoing, indirect discussions with Iran about the detainees, the two issues are not connected, the official said, and “there is no deal.”