VIENNA (AP) — Negotiators gathered in Vienna on Monday to resume talks over reviving Iran’s 2015 nuclear deal with world powers, with hopes of quick progress muted after the arrival of a hard-line new government in Tehran led to a more than five-month hiatus.

The remaining signatories to the nuclear deal formally known as the Joint Comprehensive Plan of Action — Iran, Russia, China, France, Germany and Britain — will convene at the Palais Coburg, the luxury hotel where the agreement was signed six years ago. The talks come as Austria remains locked down over the coronavirus, which start a week earlier over a surge in cases.

The last round of talks, aimed at bringing Iran back into compliance with the agreement and paving the way for the U.S. to rejoin, was held in June. Since then, the task has only become more difficult.

The U.S. is not at the table because it unilaterally pulled out of the deal in 2018 under then-President Donald Trump, who restored and augmented American sanctions in a campaign of “maximum pressure” to try to force Iran into renegotiating the pact.

President Joe Biden has signaled that he wants to rejoin the deal. A U.S. delegation headed by the administration’s special envoy for Iran, Robert Malley, is participating indirectly in the talks, with diplomats from the other countries acting as go-betweens.

The nuclear deal saw Iran limit its enrichment of uranium in exchange for the lifting of economic sanctions. Since the deal’s collapse, Iran now enriches small amounts of uranium up to 60% purity — a short step from weapons-grade levels of 90%. Iran also spins advanced centrifuges barred by the accord and its uranium stockpile now far exceeds the accord’s limits.

Iran maintains its atomic program is peaceful. However, U.S. intelligence agencies and international inspectors say Iran had an organized nuclear weapons program up until 2003. Nonproliferation experts fear the brinkmanship could push Tehran toward even more-extreme measures to try and force the West to lift sanctions.

Making matters more difficult, United Nations nuclear inspectors remain unable to fully monitor Iran’s program after Tehran limited their access. A trip to Iran last week by the head of the International Atomic Energy Agency, Rafael Grossi, failed to make any progress on that issue.

Russia’s top representative, Mikhail Ulyanov, said he held “useful” informal consultations with officials from Iran and China on Sunday. That meeting, he said, was aimed at “better understanding (…) the updated negotiating position of Tehran.“ Enrique Mora, the European Union official chairing the talks, on Twitter wrote Monday of “intense preparatory work ongoing.”

A delegation appointed by new President Ebrahim Raisi is joining the negotiations for the first time. Iran has made maximalist demands, including calls for the U.S. to unfreeze $10 billion in assets as an initial goodwill gesture, a tough line that might be an opening gambit.

Ali Bagheri, an Iranian nuclear negotiator, told Iranian state television late Sunday that the Islamic Republic “has entered the talks with serious willpower and strong preparation.” However, he cautioned that “we cannot anticipate a timeframe on the length of these talks now.”

Iranian Foreign Ministry spokesman Saeed Khatibzadeh meanwhile suggested Monday that the U.S. could “receive a ticket for returning to the room” of the nuclear talks if it agrees to “the real lifting of sanctions.” He also criticized a recent opinion piece written by the foreign ministers of Britain and Israel that pledged to “work night and day to prevent the Iranian regime from ever becoming a nuclear power.”

In an interview with NPR broadcast Friday, U.S. negotiator Malley said signs from Iran “are not particularly encouraging.”

Russia’s Ulyanov said there’s pressure to get the process moving after “a very protracted pause.”

“The talks can’t last forever,” he tweeted on Sunday. “There is the obvious need to speed up the process.”