Negotiators in Vienna resumed talks Monday 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 — convened at the Palais Coburg, the luxury hotel where the agreement was signed six years ago.

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The talks came as Austria is a week into a lockdown imposed because of a surge of coronavirus cases. Iran's state-run IRNA news agency reported their start, without elaborating, as journalists remained outside of the hotel.

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

The US 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 US 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 percent purity — a short step from weapons-grade levels of 90 percent. 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, US 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.

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