Tensions between Iran and the International Atomic Energy Agency (IAEA) have repeatedly flared since a 2015 deal aimed at curbing Tehran's nuclear programme in exchange for sanction relief has been left in tatters.

In the report, IAEA head Rafael Grossi said that "public statements made in Iran regarding its technical capabilities to produce nuclear weapons" have only increased "concerns about the correctness and completeness of Iran's safeguards declarations".

In recent years, Iran has reduced its cooperation with the IAEA by deactivating surveillance devices for monitoring the nuclear programme and barring inspectors, among other measures.

Ahead of an IAEA board of governors meeting next week, Grossi reiterated his call on Tehran to "cooperate fully and unambiguously with the agen

"Only through constructive and meaningful engagement can these concerns be addressed," Grossi said in the confidential quarterly report.

Tehran denies seeking to acquire nuclear weapons.

But statements by officials and politicians about the country's technical capabilities have strained an already fraught relationship, a diplomat source said.

EU-mediated efforts to revive the deal, which would have brought Washington back on board and Iran back into compliance, collapsed in the summer of 2022.

- Breaching limits -

Iran has significantly ramped up its nuclear programme in recent years and now has enough material to build several atomic bombs.

In a separate confidential report seen by AFP, the agency said that Iran's estimated stockpile of enriched uranium had reached more than 27 times the limit set out in the 2015 accord.

Iran's total enriched uranium stockpile was estimated at 5,525.5 kilogrammes as of February 10, up by 1,038.7 kilogrammes from October, the report said.

Nuclear weapons require uranium enriched to 90 percent, while enrichment to 3.67 percent -- permitted under the deal -- is enough for nuclear power stations.

Iran's stockpile also includes 712.2 kilogrammes of uranium enriched to up to 20 percent and 121.5 kilogrammes enriched to up to 60 percent, down by 6.8 kilogrammes from the last report, due to material being partially diluted.

Asked about the decision by Tehran to "down-blend" some of its stockpile of near-weapons-grade uranium, the diplomat alluded to potential political motivations.

Iran "may not want to increase the tension. They (may) have some agreement with somebody," the diplomat said, adding that Tehran was still producing about nine kilogrammes of uranium enriched to up to 60 percent purity per month.

- US 'seriously concerned' -

Amid heightened Middle East tensions, Western powers have been reluctant to get tough on Tehran for fear of spurring a wider conflict.

The IAEA's board of governors has condemned Iran's lack of cooperation as the country walked back on various commitments.

But since November 2022 the board members have refrained from tabling a resolution.

Reacting to the reports, the United States said it was "seriously concerned about Iran's continued expansion of its nuclear programme in ways that have no credible civilian purpose", State Department spokesman Matthew Miller said in Washington.

According to Eric Brewer of the US research institute Nuclear Threat Initiative (NTI), the "bigger picture continues to be extremely concerning".

Moreover, "there doesn't appear to be a viable diplomatic pathway for rolling the program back any time soon," he told AFP.

The IAEA had warned in November that Iran's "unprecedented" move to bar several of its inspectors had "directly and seriously affected" the UN body's work.

In Monday's report, Grossi said he "deeply regrets" that Iran had not reversed its decision to ban the inspectors.

Tehran in September withdrew the accreditation of eight top inspectors of French and German nationality, according to a diplomatic source.

Faced with increased criticism, the Iranian government announced last week that it had invited Grossi to visit Tehran in May for an international energy conference.