"Important and significant issues remain regarding the removal of sanctions that have not made an agreement possible so far," foreign ministry spokesman Saeed Khatibzadeh said at a news conference.

He reiterated calls for guarantees that the US will not withdraw from the 2015 deal, which was derailed in 2018 when the administration of then-president Donald Trump unilaterally pulled out of the pact.

Khatibzadeh's remarks came as the eighth round of negations to restore the agreement -- known as the Joint Comprehensive Plan of Action (JCPOA) -- has been on pause since Friday, with the negotiators returning to their capitals for consultations.

The European Union's coordinator for the talks, Enrique Mora, said at the time that "political decisions" are required to break the deadlock, and that talks are expected to resume this week.

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

The 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 reimposed severe economic sanctions after the 2018 withdrawal, prompting Tehran to begin rolling back on its commitments under the deal.

Khatibzadeh nonetheless noted "very significant progress" over the previous three weeks in negotiations over the lifting of sanctions and nuclear commitments, as well as obtaining guarantees.

"We suggest that after returning from their capitals, (other parties) come with necessary decisions so that we can conclude quickly what has been prepared in drafts," he said.

"The other parties know the differences clearly. They need to make political decisions, especially in Washington," he added.

"We await political decisions by Washington upon the return of the US delegation."

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.

"We can reach a lasting, reliable and good agreement the day after the return of the negotiators to Vienna" if the other parties make the "right" political decisions, he added.