Iran said on Monday that Washington had to make a "political decision" regarding lifting sanctions on the Islamic Republic as Tehran's demand for their full removal to revive a 2015 nuclear deal with world powers was non-negotiable.
After eight rounds of indirect talks between Tehran and Washington since April, differences remain about the speed and scope of lifting sanctions on Tehran, including Iran's demand for a US guarantee of no further punitive steps, and how and when to restore curbs on Iran's atomic work.
The talks paused on 28 January as top negotiators returned to their respective capitals for consultations. US Special Envoy for Iran Robert Malley on Sunday said he would soon return to Vienna, insisting the pact could still be revived.
"The issue of removal of sanctions and Iran benefiting from it is Iran's red line in the talks," Iranian foreign ministry spokesperson Saeed Khatibzadeh told a weekly a news conference.
"If the US returns to Vienna with a political decision and a specific agenda ... to remove the sanctions, then surely it will be possible to reach a deal quickly."
Iran's lead nuclear negotiator Ali Bagheri Kani would return to Vienna on Tuesday, Khatibzadeh said.
US President Joe Biden's administration on Friday re-introduced sanctions waivers for Iran to allow international nuclear cooperation projects, as indirect American-Iranian talks on a 2015 international nuclear agreement with Tehran enter the final stretch.
"Washington has decided to take a step which has no impact on Iran's economic situation," Khatibzadeh said, echoing Iran's Foreign Minister Hossein Amirabdollahian, who on Sunday described the US move as a "good but insufficient" step.
Iran has gradually violated nuclear limits of the nuclear pact in reaction to then-US President Donald Trump's 2018 withdrawal from the agreement between Tehran and six powers and reimposing of crippling sanctions on Iran.
Iran insists on the immediate removal of all Trump-era sanctions in a verifiable process. Washington has said it would remove curbs inconsistent with the 2015 pact if Iran resumed compliance with the deal, implying it would leave in place others such as those imposed under terrorism or human rights measures.
Khatibzadeh rejected warnings by Western officials that time is running out to resurrect the pact, given Tehran's nuclear advances.
"We do not consider any fake deadlines ... This round can be the last round if other parties fulfill their obligations and provide guarantees to Iran," he said.