Iran condemned as "not fair" Tuesday a report by the UN nuclear watchdog on traces of nuclear material found at three undeclared sites.

The comments came with talks deadlocked since March on reviving a 2015 nuclear agreement between Tehran and world powers.

"Unfortunately, this report does not reflect the reality of the negotiations between Iran and the IAEA," Foreign Ministry spokesman Saeed Khatibzadeh told reporters, referring to the Monday report by the International Atomic Energy Agency.

"It's not a fair and balanced report," he said, adding: "We expect this path to be corrected."

In the report, the watchdog said it still had questions which were "not clarified" regarding nuclear material previously found at three sites -- Marivan, Varamin and Turquzabad -- which had not been declared by Iran as having hosted nuclear activities.

It said its long-running efforts to get Iranian officials to explain the presence of nuclear material had failed to provide answers to its questions.

Iran and the IAEA agreed in March on an approach for resolving the issue of the sites, one of the remaining obstacles to reviving the 2015 deal. IAEA chief Rafael Grossi is due to "report his conclusions" to the watchdog's board of governors at a meeting scheduled for next week.

Formally known as the Joint Comprehensive Plan of Action, the 2015 deal gave Iran relief from crippling economic sanctions in exchange for curbs on its nuclear activities.

Then-US president Donald Trump unilaterally pulled out of the pact in 2018 and reimposed biting sanctions, prompting Iran to begin rolling back on its own commitments.

President Joe Biden's administration says it favours a return to the deal, including lifting key sanctions, but has rejected an Iranian demand to reverse Trump's designation on a terrorism blacklist of the elite Revolutionary Guards.

State Department spokesman Ned Price said that the United States had "full faith and confidence in the IAEA" and Grossi.

"Iran must fully cooperate with the IAEA without further delay," he told reporters in Washington.

The parties to the pact with Iran saw it as the best way to stop it from building a nuclear bomb -– a goal Tehran has always denied.

 Iran sees Israeli hand

While most of the activities discussed in the IAEA report are thought to date back to the early 2000s, sources say that one of the sites, in the Turquzabad district of Tehran, may have been used for storing uranium as recently as 2018.

Iran saw an Israeli hand in the IAEA's latest findings.

"It is feared that the political pressure exerted by the Zionist regime and some other actors has caused the normal path of the agency's reports to change from technical to political," Khatibzadeh said.

Israel on Tuesday accused its arch-foe Iran of stealing classified documents from the IAEA to hide evidence of its nuclear programme.

Israel is adamantly opposed to the 2015 nuclear deal and any effort to restore it.

"Iran stole classified documents from the UN's Atomic Agency IAEA and used that information to systematically evade nuclear probes," Israeli Prime Minister Naftali Bennett wrote on Twitter.

"How do we know? Because we got our hands on Iran's deception plan," Bennett wrote. His tweet included a link to eight files of documents in English and Farsi, as well as photographs.

The files were part of a cache allegedly taken by Israeli agents from an Iranian warehouse in 2018.

Iran's representative to the IAEA, Mohammad Reza Ghaebi, said earlier that the IAEA's report "does not reflect Iran's extensive cooperation with the agency".

"The agency should be aware of the destructive consequences of publishing such one-sided reports."

 Feud over reviving deal

In a separate report published Monday, the IAEA estimated that Iran's stockpile of enriched uranium had grown to more than 18 times the limit agreed in the 2015 deal.

Iran seeks the lifting of all sanctions that followed Trump's 2018 pullout.

"The pause in the negotiations is due to the US not giving an answer to the initiatives proposed by Iran and Europe," Khatibzadeh said.

Price replied that the United States was ready to return "immediately" to the agreement and that -- for now -- it continued to be "in our national interest to do so".

"It is ultimately up to Iran to decide to drop demands that go beyond the JCPOA and to engage in good faith," he said.