The United States has provided Iran with examples of three categories of existing sanctions — those it would need to lift to return to compliance with the Iran nuclear deal, those it would not lift, and a third category of “difficult cases” it is still working to decide, a senior State Department official said Wednesday.

During a second round of talks that ended earlier this week in Vienna, U.S. and European negotiators said discussions of possible “sequencing” of steps to be taken by each side are no longer on the agenda as the United States seeks to rejoin the accord.

Negotiations are now focused on reaching agreement on a full list of actions each side is prepared to take to comply with the terms of the 2015 agreement.

“The parties are not going to agree to anything until they see the full picture,” said the senior official, speaking on the condition of anonymity under rules set by the State Department. Using a phrase often uttered during negotiations for the original Joint Comprehensive Action Plan, or JCPOA, the official said that “nothing will be agreed until everything is agreed.”

U.S. and European officials said that progress had been made, but there were no substantive breakthroughs. “This week was much more specific,” said one Western diplomat who also spoke on the condition of anonymity to discuss sensitive negotiations. “There are so many things that need to be resolved. I think at this very interim beginning step it is: What can we put on the table to show good faith and confidence building?”

Negotiators have returned to their capitals for consultations and expect to resume talks in Vienna next week. Participants include Britain, France, Germany, Russia, China and the European Union, all signatories to the original agreement along with Iran and the United States.

President Donald Trump withdrew from the accord three years ago, reimposed all the sanctions lifted under its original terms, and added more than 1,500 measures. In response, Iran has sharply expanded its nuclear activities beyond the limits of the deal. It has said all U.S. sanctions must be lifted before it returns to compliance.

President Biden campaigned on a U.S. return to the deal. Iran has refused to meet directly with U.S. negotiators, led by special envoy Robert Malley, but agreed to indirect talks. The United States’ European allies in the group have shuttled back and forth between the two to answer questions and convey negotiating terms.

What Iran needs to do is fairly straightforward. The goal of the original deal was to limit Iran’s activities to ensure it would not build a nuclear weapon, and impose strict inspections and verification by the International Atomic Energy Agency. About a year after Trump’s withdrawal and the imposition of “maximum pressure” sanctions designed to cripple Iran’s economy, Tehran began moving far beyond those limits, enriching more uranium to higher levels.

Iran's return to the JCPOA limits is seen as fairly simple, although there is concern about additional knowledge Iranian scientists have accumulated through accelerated enrichment activities.

For the United States, compliance is more difficult. “It’s kind of like a Rubik’s cube,” the Western diplomat said. The JCPOA spoke only to “nuclear-related sanctions” and Iran’s right to participate in the international economic system, and that is theoretically the point to which the United States would have to return.

A key difficulty in sorting out the “third category” of “difficult cases,” the State Department official said, is that “the Trump administration deliberately and avowedly imposed sanctions involving the terrorist label . . . even though they were done purely for the purpose of preventing or hindering” the U.S. return to compliance with the nuclear deal.

Although the official declined to provide numbers in each of the three categories or cite examples, others have pointed to Trump’s sanctioning of Iran’s Central Bank for terrorism as one likely to be lifted, while measures imposed against the military Islamic Revolutionary Guard Corps are likely to remain.

“It’s a work in progress,” the official said of U.S. compilation of the baskets. “We understand that lifting of some sanctions will have to come from a return to the JCPOA,” the official said. “But that doesn’t mean that we can’t continue to counter Iranian activities that are destabilizing and go against our interests or those of our partners.”

Other obstacles also remain, including domestic political pressures on both sides. “Both Iran and America have an amazing quantity of hard-liners in parliament,” the Western diplomat said.

One illustration came Wednesday, when former secretary of state Mike Pompeo made his first appearance on Capitol Hill since Biden’s inauguration. Pompeo spoke at a news conference at which congressional Republicans unveiled what they called the “Maximum Pressure Act,” aimed at preserving all existing sanctions against Iran and adding more.

Pompeo played a key role in Trump’s efforts to isolate Iran. The legislation, sponsored by the Republican Study Committee, is designed to limit Biden’s efforts at rejoining the nuclear deal.

The bill would also mandate congressional approval of any agreement Biden made with Iran.

Morris reported from Berlin.