“The question still remains about whether the seriousness of purpose and the intent of coming back into compliance that the U.S. showed will be reciprocated by Iran,” the senior State Department official said in a conference call with reporters. “We saw some signs of it, but certainly not enough.“

The talks between the U.S. and Iran, being conducted indirectly in Vienna using European intermediaries, were paused Friday as teams from both sides return to their capitals for consultations. The discussions are slated to resume next week, with key issues of what each country must do, and in what order, still unresolved.

The 2015 deal was negotiated under former President Barack Obama's administration, and it involved several countries as well as assistance from the European Union and the United Nations. It lifted an array of U.S. and international nuclear-related sanctions on Iran in exchange for severe curbs on Tehran’s nuclear program.

In 2018, citing many of the complaints critics level at the deal today, Trump walked away from it. He reimposed the sanctions lifted under the agreement as well as tacked on new ones. Over time, Iran, in retaliation, began resuming some of its nuclear activities, including enriching uranium to 20 percent purity.

The team President Joe Biden dispatched to Vienna, led by special envoy Rob Malley, has been looking at various options for returning to the agreement, which is often referred to as the JCPOA based on its official name, the Joint Comprehensive Plan of Action.

Malley’s team has been sifting through the numerous sanctions imposed and re-imposed on Iran during the Trump years. While many of the sanctions are clearly aimed at Iran’s nuclear program, Trump aides intentionally categorized others as falling under other headings, such as punishing Iran over its human rights record, its support for terrorism or its ballistic missile program. The Biden team has to decide which sanctions it believes were legitimately categorized, and should be kept, and which ones were a veiled attempt to sanction Iran over its nuclear program, and should be lifted if the U.S. returns to the deal.

The senior State Department official pointed out that the original nuclear deal allowed the United States to sanction Iran on grounds that were not nuclear-related. If the Biden team decides that a Trump era sanction was legitimately imposed on those other grounds, it is not bound to lift the sanction.

“There are some that are legitimate sanctions even under a very fair reading, scrupulous reading“ of the deal, the senior State Department official said, declining to offer specifics.

On Friday, Zarif tweeted: “All Trump sanctions were anti-JCPOA & must be removed — w/o distinction between arbitrary designations.” The senior State Department official said the tweet was unhelpful and suggested that the Iranians were not serious about reviving the deal.

Zarif also tweeted that the United States must take the first step by removing sanctions, because it caused the crisis when Trump left the deal. He added that Iran will make its moves after “rapid verification,” an apparent reference to Iran checking to see if the sanctions removal has taken effect.

Thanks to the existence of organizations such as the International Atomic Energy Agency, there are ways to verify that Iran has stopped its nuclear activities. But Iranian officials haven’t laid out what would suffice for them when it comes to verifying the sanctions lifting. Depending on what exactly the Iranians mean, that could get tough, especially if Tehran is seeking proof that lifting the sanctions is having an effect on its economy.

“We don’t know exactly what they mean” on sanctions verification, the senior State Department official said. “We’d welcome more details on precisely what they have in mind.”

Ali Vaez, a top Iran analyst with the International Crisis Group who has contacts on both the U.S. and Iranian sides, said the sanctions verification could get very complicated.

“When it gets to the money being in their bank accounts, I understand how you verify that,” Vaez said. “But what if a South Korean company that has already gone through the very expensive process of switching to another kind of oil doesn’t want to switch back to Iranian oil? Is that the U.S.’s fault? Would this be a sign of bad faith? It’s difficult to answer.”

Who takes which steps first also is a tricky issue, but not impossible to figure out, Vaez and others said. It could come down to how the moves are described. It may have to be “designed creatively so that it’s a step-by-step process so that it looks like one step,” Vaez said. “It sounds like they’ll try to define it like one step with different sections.”

For now, the Biden team appears intent on restoring the original deal, and it’s largely ignoring the outside criticism. But the volume of that criticism is likely to rise in the coming days as Tehran and Washington edge closer to an arrangement.

“We cannot go back to the dangerous nuclear plan, because a nuclear Iran is an existential threat and a very big threat to the security of the whole world,” Israeli Prime Minister Benjamin Netanyahu, one of the most prominent opponents of the nuclear deal, said Tuesday. Trump cited Israeli arguments against the nuclear deal when he quit it in May 2018.

Netanyahu followed up Wednesday with a pointed warning to the United States: “To our best friends I say — an agreement with Iran which paves its way to nuclear weapons that threaten us with destruction — an agreement like this will not bind us.” It was an indication that Israel intends to keep up its own efforts to undermine Iran on the nuclear front, which have included assassinations of Iranian scientists.

Israel is suspected of being behind a mine explosion that damaged an Iranian ship in the Red Sea on Tuesday, according to media reports. The New York Times, citing an unnamed American official, reported that Israel had told the U.S. that the blast was retaliation for earlier Iranian strikes on Israeli vessels. Israel has not publicly confirmed or denied a role in the apparent attack.

The international back-and-forth comes on top of domestic U.S. criticism of a return to the deal, led by Republicans as well as some Democrats.

“The long slide toward surrender begins,” tweeted John Bolton, a former Trump administration national security adviser, alongside a link to a story about the talks.

Several senators have signed on to letters condemning the idea of returning to the agreement.

The letters, including one sent this week, urge the Biden administration to keep sanctions on Iran, arguing they provide the U.S. leverage to help shape Iranian behavior. They argue that the 2015 deal had too many provisions that expire and should have covered Iranian actions outside of the nuclear sphere, such as its support for terrorism and proxy militias.

“We oppose any attempt to return to the failed JCPOA, or any deal that offers one-sided concessions to the Iranian regime while it continues to undermine the security of the United States and our allies and partners,” four GOP senators wrote in one missive.

Even some lawmakers who weren’t in Congress when the original deal was unveiled are now voicing their disapproval.

Democratic Rep. Elaine Luria of Virginia, who was elected to Congress in 2018, tweeted: “I have serious reservations about re-starting negotiations with Iran while they continue to enrich uranium at dangerous and unacceptable levels.”

The deal’s supporters say the critics are being intellectually dishonest.

For instance, they note that the point of negotiating a return to the deal is to make Iran end activities like higher level enriching of uranium. And when it comes to sanctions, even if the U.S. once again lifts those that had been lifted by the 2015 deal, there still will be numerous other U.S. sanctions that remain on Tehran.

The Biden administration has, however, said it wants to negotiate a “longer and stronger” deal with Iran, which could address issues beyond the nuclear front, such as Iran’s ballistic missile program. But the administration insists that the first step should be returning to the original nuclear agreement.

The Biden administration’s desire to rejoin the agreement has supporters in Congress, too.

A person familiar with the issue confirmed that Democratic Sens. Chris Murphy of Connecticut and Tim Kaine of Virginia are circulating a letter to obtain signatures from colleagues. The letter “specifically supports a compliance [for] compliance return to the JCPOA and also urgently addressing other regional security issues,” the person said.

The senior State Department official said the Biden administration is well aware of the intense political interest on Capitol Hill. The official said the administration has been in touch with lawmakers and will continue to do so even when it disagrees with their position.

The official added that one aspect of the talks that has been complicated is their indirect nature. The United States would be happy to meet directly with Iranian officials, but so far the Iranians have not indicated they’re ready to sit at the same table.