The International Atomic Energy Agency (IAEA) did not detail what the questions related to but said it was "discussing substance" with the Iranians.

There has been "engagement" from Tehran in recent weeks on questions relating to its nuclear safeguards declarations to the agency, IAEA acting head Cornel Feruta told journalists in Vienna.

"(That) engagement doesn't mean that the issues are completely addressed but it's a step in the right direction," he added.

Feruta said the queries did not touch directly on the faltering 2015 nuclear deal between Iran and world powers but rather on Iran's separate safeguards agreement with the agency.

Diplomatic sources in Vienna say the agency has been waiting for information from the Iranians relating to samples taken earlier this year from a warehouse near the capital Tehran.

In 2018, Israeli Prime Minister Benjamin Netanyahu accused Iran of using the site for undeclared nuclear activity.

At the beginning of September, Feruta had meetings with high-level officials in Tehran and subsequently urged Iran to "respond promptly" to the IAEA's questions, adding: "Time is of the essence."

On Friday, he said the IAEA had "been having a very active dialogue with Iran at both technical and higher levels in the past few weeks."

Feruta met Iran's atomic energy agency chief Ali Akbar Salehi during the IAEA's General Conference in Vienna in late September, followed by meetings with Foreign Minister Mohammad Javad Zarif and his deputy Abbas Araghchi during the UN General Assembly in New York.

"I think the message we sent out in September and also during my visit to Tehran was understood by the Iranian officials and as a result, we have this engagement on these particular issues," Feruta said.

The latest developments come a week after the IAEA reported that Iran had started using advanced models of centrifuges to enrich uranium, a fresh breach of the 2015 deal.

Iran has been progressively abandoning commitments under the deal in response to the US withdrawal from the pact and re-imposition of sanctions.