TEHRAN: The United Nations nuclear watchdog said there had been "no progress" in talks with Tehran on disputes over the monitoring of Iran's atomic program, just days before talks restart on reviving the 2015 Iran nuclear deal.

Rafael Grossi, director general of the International Atomic Energy Agency (IAEA), told a quarterly meeting of the agency's board that talks he held in Tehran on Tuesday were "inconclusive" despite being "constructive."

Grossi had sought to tackle constraints put on IAEA inspections earlier in 2021, outstanding questions over the presence of undeclared nuclear material at sites in Iran and the treatment of IAEA staff in the country.

"In terms of the substance...we were not able to make progress," the director general told reporters, saying that the lack of agreement had come "inspite of my best efforts."

Behrouz Kamalvandi, spokesman for the Atomic Energy Organization of Iran, told Iranian television his team "tried until the last moment" but there is still work to be done.

Among other officials in Tehran, Grossi met Foreign Minister Hossein Amir-Abdollahian.

Amir-Abdollahian put a positive gloss on the talks, telling the official IRNA news agency on Wednesday that a "common declaration" had been reached which would be published "as soon as possible."

Grossi's visit came ahead of the scheduled resumption on Monday of negotiations between Tehran and world powers aimed at reviving the 2015 deal that gave Iran sanctions relief in return for curbs on its nuclear program.

The US said that it was "disappointed" by the outcome of Grossi's visit to Iran and said it was ready to negotiate in Vienna.

The remaining members of the accord — France, Germany, the UK, China, Russia and Iran — will attend with the US taking part indirectly.

The deal has been gradually disintegrating since former US President Donald Trump unilaterally withdrew from the deal in 2018.

The following year Iran retaliated by starting to move away from its commitments under the deal, also known as the JCPOA — Joint Comprehensive Plan of Action.

The US negotiator for the JCPOA talks Rob Malley warned that Washington would not "sit idly" if Iran delayed progress at the talks.

"If [Iran] continues to do what it appears to be doing now, which is to drag its feet at the nuclear diplomatic table and accelerate its pace when it comes to its nuclear program...we'll have to respond accordingly," Malley told US broadcaster NPR.