"What remains are important and key issues that require specific political decisions," foreign ministry spokesman Saeed Khatibzadeh said at his weekly news conference.

"Washington, in particular, must announce its decisions on removing sanctions and the remaining issues," he said.

"If this happens ... we will reach a lasting, reliable agreement at a good pace."

His remarks came on the day chief negotiators were due to return to Vienna for the talks after having travelled home on Saturday for consultations.

Discussions continued over the weekend at the level of experts.

The drive to salvage the 2015 nuclear deal resumed in late November, after talks were suspended in June as Iran elected ultraconservative President Ebrahim Raisi.

The 2015 deal -- agreed by Iran, the US, China, Russia, Britain, France and Germany -- offered Tehran sanctions relief in exchange for curbs on its nuclear programme.

But the US unilaterally withdrew in 2018 under then-president Donald Trump and reimposed stiff economic sanctions, prompting Tehran to begin rolling back on its commitments.

"Negotiations are going in the right direction, we have no unresolvable impasses," Khatibzadeh said, pointing to consensus over much of the text of a new deal.

Different parties have signalled progress at the talks, with Tehran saying this was due to efforts by "all sides".

But Khatibzadeh criticised delays, "especially by the US", in proposing initiatives for the lifting of sanctions.

The US has participated only indirectly in the talks, which seek to bring Washington back to the accord and to ensure Iran returns to its commitments under the deal.

Western parties have insisted on the "urgency" of reaching a deal, with US Secretary of State Antony Blinken saying on Thursday that there are only weeks left to save the accord.

Washington is ready to look at "other options" if negotiations fail, he said.

In response, Khatibzadeh warned: "Mr Blinken knows better than everyone that every country has its own Plan B and Iran's Plan B might not be in the interest of the US."