Iran and world powers resumed talks Monday aimed at reviving the 2015 Iran nuclear deal after a five-month break in negotiations, during which a new government in Tehran hardened its position.

After a meeting lasting about two hours, Enrique Mora, the European Union official chairing the talks, emerged to call the session “extremely positive,” with all participants showing “a sense of urgency” in bringing the deal back to life.

But Ali Bagheri, Iran’s new chief negotiator, stressed that his country’s position remains that all sanctions against it imposed since President Donald Trump withdrew from the agreement in 2018 must be lifted as a first order of business. “It was agreed that first, the issue of the removal of the sanctions . . . be set as the prime agenda,” Bagheri said, according to Iranian media reports. A follow-up meeting of the negotiating working group on sanctions is scheduled for today. A separate group on Iran’s nuclear program is to convene later in the week.

Under the original accord, called the Joint Comprehensive Plan of Action, or JCPOA, the United States agreed to lift economic sanctions against Iran in return for sharp limits and international monitoring of Iran’s nuclear activities.

After withdrawing, Trump imposed some 1,500 sanctions as part of his “maximum pressure” campaign against Iran. Iran waited about a year, and then began rebuilding its nuclear program at an increasingly rapid pace. The Biden administration now estimates that the amount of time the Iranian program could reach “breakout” — how long it takes to produce enough fissile material for one bomb — has shrunk from one year under the deal to less than a month.

Iran has said it is not interested in building a nuclear weapon.

The administration has said repeatedly that it is seeking “compliance for compliance” in returning to the original accord, and will lift all nuclear-related sanctions. But other Trump-era measures addressed Iran’s proxy wars in the region and alleged support for terrorism and are not on the U.S. list for removal.

In an opinion article appearing in Sunday’s Financial Times, Bagheri said that the United States, as the party that left the deal in the first place, must “prioritize compensation” for its violation, “which includes the removal of all post-JCPOA sanctions.”

Iran is also seeking a U.S. guarantee that no future administration will again withdraw, a guarantee that is politically impossible for President Biden or any other president.

“We have made our choice,” Bagheri wrote. “We will now find out whether or not the west has the will to enter real negotiations.”

At Monday’s Vienna meeting, diplomats from the five countries party to the JCPOA — Britain, France, Germany, Russia and China — sat across the table from Bagheri and his team. Iran has refused direct talks with the United States, and the U.S. team headed by chief negotiator Robert Malley received a readout from European partners at a separate Vienna hotel.

Negotiations with Iran’s previous government began in April. Over six sessions ending in June, the two sides had made progress on steps each would take toward compliance, and had begun to address questions of sequencing under a final accord, according to U.S. and European officials.

Since then, Iran has deployed ever more advanced centrifuges, enriching greater quantities of uranium to levels far beyond the limits of the JCPOA. Under the new government headed by President Ebrahim Raisi, it has refused to supply International Atomic Energy Agency monitors, tasked by the JCPOA with verifying its program, access to an important centrifuge manufacturing facility and to respond to questions about uranium traces found at three undeclared Iranian sites.

In response, the United States and its European allies have upped the ante by publicly warning that their patience is not infinite, and referring to a “Plan B” for responding to an untethered Iranian program. While not ruling out future military action, the administration has indicated the likely first response to failure would be economic.

But “unquestionably, our best approach here is through diplomacy,” White House press secretary Jen Psaki told reporters Monday. “Our objective has not changed — it remains a mutual return to full compliance.”

The administration has spent considerable diplomatic energy trying to keep all members of the JCPOA on board. Russia and China, whose relations with the United States have become increasingly strained, have publicly backed a return to the deal while disapproving of economic sanctions and strengthening their own ties with Iran.

In the Middle East, Persian Gulf states that strongly objected to the pact when it was signed by the Obama administration still view Iran as a threat, but have lost confidence in the U.S. commitment and ability to restrain it. Saudi Arabia and the United Arab Emirates have begun to hedge their bets by beginning tentative diplomatic outreach to Tehran.

Israel and Iran remain staunch enemies. In a statement before the talks began Monday, Israeli Prime Minister Naftali Bennett said that “Iran deserves no rewards, no bargain deals and no sanctions relief in return for their brutality. I call upon our allies around the world: Do not give in to Iran’s nuclear blackmail.”

Liz Sly in London contributed to this report.