The talks, with European Union officials as intermediaries, aim to restore the bargain at the core of the agreement - restrictions on Iran's nuclear activities in exchange for the lifting of US and other worldwide sanctions. "There's still question marks about whether Iran has the willingness to... take the pragmatic approach that the United States has taken to come back into compliance with its obligations under the deal", he said.
A senior US State Department official told reporters the United States had seen some signs of Iranian seriousness about returning to the nuclear pact but "certainly not enough". US and Iranian diplomats concluded an initial round of meetings earlier in the day, with mediators from the European Union delivering messages between the two sides.
The U.S. official described this week's talks as productive, businesslike and constructive.
The "US -- which caused this crisis -- should return to full compliance first", Iranian Foreign Minister Mohammad Javad Zarif wrote on Twitter, adding that "Iran will reciprocate following rapid verification".
Iran announced Saturday it has started up advanced uranium enrichment centrifuges in a breach of its undertakings under a troubled 2015 nuclear deal, days after talks on rescuing it got underway.
US Secretary of Defense Lloyd Austin arrived in Israel on Sunday on the first visit by a senior representative of the Biden administration, whose stance on Iran has anxious Prime Minister Benjamin Netanyahu's government.
The remaining parties to the accord - Iran, Britain, China, France, Germany and Russia - met again on Friday after talks formally began on Tuesday and they agreed to keep going, Russian and Chinese envoys said.
"The United States team put forward a very serious idea and demonstrated a seriousness of objective on coming back into compliance if Iran comes back into compliance", a top US State Department official in the talks told reporters Friday.
The 2015 deal placed strict limits on Iran's uranium stockpile.
Still, he expressed Washington's openness to lift non-nuclear sanctions. The US maintained a "maximum pressure" policy towards Iran during the Trump era, crushing the Iranian economy with sanctions and severing its connection to the global financial system.
"Depending on the announcements it also could be a sign of whether they are approaching these talks in a constructive spirit, and taking, taking on board the fact that the U.S. has shown a real seriousness of objective, or whether they're going to take a different track which would raise more questions about their intention", the official said.
He declined to discuss the specifics of what sanctions can be lifted, but he noted that the administration is open to take on the complicated task of reversing some measures labelled as terrorism sanctions.
Gärtner said Iran's outgoing President Hassan Rouhani, whose government negotiated and entered the JCPOA in 2015, also has an incentive to bring sanctions relief to his people again and thereby avoid leaving office with his reputation marred by a failed agreement.
Given the complexities of the sanction and the demands by each party, the official decried Iran's objection to sitting on the same table as the US.