The head of the UN's nuclear watchdog said Monday he would continue to be "firm and fair" with Iran, a day after clinching a deal over access to surveillance equipment at Iran's nuclear facilities.

International Atomic Energy Agency (IAEA) Director General Rafael Grossi was speaking to reporters at the start of a meeting of the agency's board of governors at a delicate moment for international diplomacy on the Iranian nuclear issue.

On Sunday he returned from a visit to Tehran where he hammered out a deal to enable the IAEA to service its surveillance equipment there and to ensure footage recorded on it is preserved.

The footage will be handed over to the IAEA if and when there is an agreement between Iran and world powers on the revival of the 2015 deal, also known as the JCPOA.

Talks to revive the deal are currently stalled, with Iran warning it may be months before they restart.

Little progress has been made on another issue relating to long-standing questions the IAEA has had about the previous presence of nuclear material at undeclared sites in Iran.

The agency has said in numerous reports that Iran's explanations about the material have not been satisfactory.

Asked whether now was the time to be tougher with Iran on the issue, Grossi replied that "from day one I have had an approach with Iran which is firm and fair".

He said he hoped a "clear conversation" with the government of new ultraconservative President Ebrahim Raisi would take place on a return visit to Iran "very soon" and would bring progress.

Eyes wide shut

Raisi replaced moderate president Hassan Rouhani, whose landmark achievement was negotiating the JCPOA.

Grossi admitted that Raisi's government had "firm views on matters related to the nuclear programme" but said that nevertheless "we must engage".

"These things are not going away. We need to address them together," he added.

In the run up to this week's board of governors meeting there had been speculation that Western countries may push for a resolution censuring Iran but a diplomatic source told AFP that the deal struck over the weekend had "in principle removed" that possibility.

Iran's conservative press meanwhile on Monday celebrated the weekend's deal. 

The Javan daily said it meant "Iran had not revealed its secrets to the agency", while the Vatan-e-Emrouz newspaper titled its coverage "Eyes wide shut".

Asked how difficult it would be to reconstruct information once the IAEA gains access to the footage, Grossi admitted that "it's something that has to a certain extent never been done before but it's not... beyond the capacity of my technical teams." 

However he confirmed that the agency still has access to footage "as often as required" from sites such as Iran's enrichment plants at Natanz and Fordow.