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 this year, 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," Grossi told reporters, saying that the lack of agreement had come "in spite 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 agency on Wednesday that a "common declaration" had been reached which would be published "as soon as possible".

-'Foot-dragging'-

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 programme.

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

"But of course Iran's failure to cooperate is a bad sign about their seriousness in a successful conclusion to our negotiations," a State Department spokesperson said.

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.

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.

At the IAEA Board of Governors' meeting, the EU issued a joint statement saying it was "deeply concerned at the inconclusive outcome of the discussions" with Grossi.

Russia's representative meanwhile said it supported Grossi's "intent to continue working with the Iranian side and call for Tehran to do the same".

-'Excessively invasive'-

One of the steps away from the deal came earlier this year when Iran began restricting some IAEA inspections activity.

Iran and the agency currently have a temporary agreement that gives the IAEA access to monitoring equipment at Iran's nuclear facilities.

However, the Agency has warned that the agreement is not a durable solution and Grossi said he was "close to... the point where I would not be able to guarantee continuity of knowledge" of Iran's nuclear programme if it continued.

Grossi also said he had raised concerns while in Tehran about security checks on IAEA inspectors which the agency has described as "excessively invasive".

He noted that the IAEA and Iran had a legal agreement "which is intended to protect inspectors from intimidation, from seizure of their property".

"Our Iranian colleagues have instituted a number of measures which are simply incompatible" with this, he said.