Iran on Sunday demanded that the UN nuclear watchdog "completely" resolve outstanding issues related to questions over nuclear material at undeclared sites, amid resumed talks to revive its 2015 nuclear deal.

Talks aimed at reviving the agreement over Iran's nuclear programme resumed this week in Vienna, months after they had stalled.

Iranian sources have suggested that one of the key sticking points is a probe by the International Atomic Energy Agency on traces of nuclear material found at undeclared Iranian sites.

"We believe that the agency should completely resolve the remaining safeguard issues from a technical route by distancing itself from irrelevant and unconstructive political issues," Iran's foreign minister Hossein Amir-Abdollahian said.

During a phone call with UN Secretary-General Antonio Guterres, Amir-Abdollahian reiterated that his country is "serious about reaching a strong and lasting agreement", Iran's foreign ministry said.

"The outcome of this matter depends on whether the United States wants to make an agreement," he added.

The IAEA's board of governors adopted a resolution in June, censuring Iran for failing to adequately explain the previous discovery of traces of enriched uranium at three previously undeclared sites.

But Tehran argued on Friday that the issues surrounding the undeclared sites "are political in nature and should not be used as a pretext for abuse against Iran in the future".

"Nuclear weapons have no place in the doctrine of the Islamic republic of Iran and are contrary to our policies and beliefs," Amir-Abdollahian reiterated on Sunday.

The negotiations to revive the deal, known formally as the Joint Comprehensive Plan of Action, began in April 2021 before coming to a standstill in March.

The 2015 deal gave Iran sanctions relief in exchange for curbs on its nuclear programme to guarantee that Tehran could not develop a nuclear weapon -- something it has always denied wanting to do.

But the US unilateral withdrawal from the accord in 2018 and the reimposition of biting economic sanctions prompted Iran to begin rolling back on its own commitments.

Russia's envoy to Vienna Mikhail Ulyanov however said Sunday that talks are moving "in the right direction".

A successful conclusion can be reached "very soon, but no guarantees -- as always nothing is agreed until everything is agreed", he told reporters outside the Palais Coburg hotel where the talks are being held.