"I appeal to the United States to lift or waive its sanctions as outlined in the plan and extend the waivers regarding the trade in oil with the Islamic Republic of Iran," said Rosemary DiCarlo, the UN Under-Secretary-General for Political and Peacebuilding Affairs.

"Also important is the extension of US waivers regarding certain civilian nuclear-related activities," she told the UN Security Council, as negotiations resumed in Vienna on reviving the Joint Comprehensive Plan of Action, or JCPOA, as the nuclear deal is formally known.

She added waiver extensions were also needed for exchanging enriched uranium out of Iran for natural uranium.

The deal has been on life support since 2018, when former president Donald Trump pulled the US out of the accord, which provided Iran with sanctions relief in return for curbs on its nuclear program, prompting Tehran to begin rolling back on its commitments.

DiCarlo called on Iran "to reverse the steps it has taken that are not consistent with its nuclear-related commitments under the plan."

In a joint statement, Germany, Britain and France, all signatories of the nuclear deal along with Russia and China, said "the diplomatic door is firmly open for Iran to do a deal now."

"Iran has to choose between the collapse of the JCPOA and a fair and comprehensive deal, for the benefit of the Iranian people and nation," the statement added.

Iran's UN ambassador, Majid Takht Ravanchi, said that to resume its commitments under the nuclear agreement, his country was not "imposing any preconditions or new conditions" and that all the measures it has taken since Washington left the accord were "reversible."

His US counterpart, Linda Thomas-Greenfield, said Washington was "prepared to return to US compliance and to stay in compliance, so long as Iran does the same."

Indirect negotiations between Iran and the United States, mostly carried out by European powers, resumed at the end of November in Vienna in an attempt to resuscitate the agreement that was aimed at preventing Tehran from acquiring nuclear weapons.