The accord offered Tehran sanctions relief in exchange for curbs on its nuclear programme, but then US president Donald Trump unilaterally withdrew from it in 2018 and reimposed heavy sanctions.

Iran's Foreign Minister Hossein Amir-Abdollahian urged the US guarantee on the so-called Joint Comprehensive Plan of Action in an interview with the Financial Times published on his ministry's website.

"As a matter of principle, public opinion in Iran cannot accept as a guarantee the words of a head of state, let alone the United States, due to the withdrawal of Americans from the JCPOA," he said.

He stressed he had asked Iranian negotiators to propose to the Western parties that "at least their parliaments or parliament speakers, including the US Congress, can declare in the form of a political statement their commitment to the agreement and return to the JCPOA implementation".

The Vienna talks involve Iran as well as Britain, China, France, Germany and Russia directly and the United States indirectly. They aim to return the US to the nuclear deal, including through the lifting of sanctions on Iran, and to ensure Tehran's full compliance with its commitments.

"Iran's commitments are as clear as a mathematical formula," said Amir-Abdollahian.

"It is absolutely clear what we are supposed to do and how these measures will be verified" through the UN's International Atomic Energy Agency, he said, adding that the other parties must have "no concern".

- 'Sustainable guarantees' -

"But we remain concerned primarily about the guarantees" from the US side, he noted, adding that "we are facing problems during this period because the other party lacks a serious initiative".

The talks in the Austrian capital resumed in late November after a pause following the June election of ultraconservative President Ebrahim Raisi.

The US had announced its readiness to hold direct talks with Iran to resolve the remaining issues, but Tehran has said preconditions must be met before the two arch-rivals can sit at the negotiating table.

"We are not ready to enter into the process of direct talks with the US if we do not have a clear and promising outlook to reach a good agreement with sustainable guarantees in front of us," said the Iranian top diplomat.

If the intentions of the US are "genuine" they should take some "practical and tangible steps on the ground before any direct talks and contacts can take place," he added.

Amir-Abdollahian said any direct dialogue, contact and negotiations with Washington would have "very huge costs for my government".

US-Iran relations have been severed since April 1980, just months after the fall of the shah and the occupation of the American embassy and following hostage crisis.