The Biden administration has opened the door to lifting Trump-imposed sanctions against Iran that are "inconsistent" with the 2015 nuclear deal, and also those that prevent Iran from accessing broader economic benefits under the agreement, according to a senior State Department official.

The formulation, offered after a first round of indirect talks between Iran and the United States this week, is designed to break a roadblock between what Iran has demanded and what the administration is prepared to give for both to return to compliance with the accord.

The official described the talks as "businesslike" and the atmosphere as "very constructive." Discussions are expected to resume in the middle of next week.

At Iran's insistence, there was no direct communication between the Iranians and the U.S. delegation headed by special envoy Robert Malley. Messages were carried back and forth by other participants in the nuclear deal — Britain, Germany and France, as well as Russia and China.

"If they don't want to sit down with us, too bad for them," the official said. "It's just going to be harder for them to get what they want." The official briefed reporters on the talks on the condition of anonymity, under rules set by the State Department.

The official described the negotiations as "complicated," and said the initial round had centered on the specifics of what each side is prepared to do to return to compliance with the agreement, from which President Donald Trump withdrew three years ago.

"We did not submit a list. We exchanged ideas about the principles that would guide any sanctions on the U.S. side. Iran did not submit a list either," the official said. Since the U.S. withdrawal, Iran has increased the quantity and quality of its enriched uranium, far exceeding the limits imposed by the 2015 agreement. It has activated more-sophisticated centrifuges, all of which would have to be decommissioned and placed under verifiable seal.

But the situation is far more complicated for the United States.

While the original text lifted specific sanctions that had been imposed related to Iran's nuclear activity, it also broadly invited Iran back into the international economic community, so that it could trade and seek foreign investment without worry that the United States would threaten or impose sanctions on those conducting business with it.

Sanctions related to other aspects of Iran's behavior — alleged terrorism sponsorship, support of proxy wars and development of ballistic missiles — were not lifted by the original agreement, and not prohibited in the future.

But when Trump withdrew from the agreement, he not only reimposed all the sanctions that had originally been lifted but also added over 1,500 more. Many were not labeled as nuclear-related but as punishment for terrorism or other activities.

Included among them were Iran's Central Bank and other financial institutions, which were called terrorist entities. Trump administration officials were fairly open at the time that this was done to make it harder for a successor administration to simply return to the agreement by lifting "nuclear" sanctions.

"The United States retains the right to impose sanctions for nonnuclear reasons," the official said. "We've made clear, publicly, to the Iranians indirectly, our view is that all sanctions inconsistent with the [deal] and benefits that Iran expects" from the agreement, "we are prepared to lift if Iran comes back into compliance."

"That doesn't mean all of them," the official said of the existing sanctions, adding that "some" are "legitimate." But "our position is . . . consistent with plain language and any fair interpretation of the deal."

The administration has undergone what the official said was a "painstaking effort to determine which need to be lifted. . . . The label itself doesn't always give the answer. We have to keep in mind that there was a purposeful and self-avowed effort" by the previous administration "precisely with the political intent to make it hard for any successor that wanted to go back in to do so."

Iran's position heading into the new talks is that "all" sanctions imposed by Trump must be lifted before it will return to compliance. "If Iran sticks to the position that every sanction since 2017" must be removed, the talks will remain at an "impasse," the official said.

"All Trump sanctions were anti-JCPOA" and "must be removed," Iranian Foreign Minister Javad Zarif tweeted Friday, using the initials for the nuclear pact.

Iran also said Friday that it had released a South Korean ship seized three months ago and released its captain, easing at least one source of tension between Tehran and Washington.

Iranian forces intercepted the South Korean tanker in the Persian Gulf in January, alleging it was captured for “technical” reasons related to environmental pollution, while also complaining that Seoul had frozen $7 billion of its assets to comply with U.S. sanctions.

It was unclear to what extent Iran’s move was linked to the talks in Vienna. But Iran’s demand for access to its frozen funds is part of broader negotiations over the revival of the nuclear deal between Tehran and six world powers.

Iran has in the past strongly hinted that access to its funds in South Korea might be the kind of goodwill gesture that would break the deadlock with the United States. The money would pass through Switzerland and be restricted to purchases of humanitarian goods such as medical devices, Iranian officials have said.

A South Korean Foreign Ministry official said Friday that “we firmly expressed our willpower to solve issues related to the frozen funds.” The official, who was not authorized to be named, said South Korea’s “efforts were communicated to Iran and could have positively influenced the final decision to release the seized vessel.”

But the State Department, while saying it was “pleased” with release of the ship, said in a statement that “the United States has not authorized the release of the Iranian funds frozen under sanctions and being held in South Korean banks.”

The European Union, which is hosting the talks, said in a statement that Iran and the other signatories to the deal — excluding the United States — held formal meetings Friday that “took stock of the discussions held at various levels” over the past days. It said that European coordinators would continue separate contacts with all participants until they reconvene next week.

Morris reported from Berlin, Denyer from Tokyo and Fahim from Istanbul. Michael Birnbaum in St. Louis contributed to this report.