In March, Iran agreed to reconnect surveillance cameras at several nuclear sites almost a year after they were turned off, as the Islamic republic has stepped up its atomic programme despite a landmark 2015 deal.

The deal -- curbing Iran's nuclear programme in exchange for sanctions relief -- started to fall apart in 2018 when the US unilaterally withdrew from it and reimposed sanctions. Efforts to revive it have been fruitless so far.

Grossi said "implementing a number of voluntary additional monitoring and verification measures" as agreed in March was "going very slow".

"There is a lot more that needs to be done... We need to go faster," he told reporters after opening a regular meeting of the 35-member board of governors of the International Atomic Energy Agency (IAEA).

Earlier in his statement to the board, he said "some progress has been made, but not as much as I had hoped", adding that what had been done so far was "a fraction of what we envisaged".

In a report to the board, seen by AFP last week, the IAEA noted that Iran has significantly increased its stockpile of enriched uranium in recent months, continuing its nuclear escalation.

The agency, however, noted progress in its cooperation with Iran in a separate report saying it has decided to, for now, close the file on nuclear material at an undeclared site.

The issue of the material found at Marivan in Abedeh county has long exacerbated relations between the two parties.

Grossi said Iran's explanation -- that the particles could be from Soviet-era mining conducted there -- was "plausible, not impossible".

The IAEA previously assessed that "there have been a number of explosives experiments in the past", he reiterated.

When asked about criticism of Iran arch-foe Israel on closing the file, Grossi insisted his agency had not bowed to any pressures.

"We never, ever water down our standards, we stand by our standards," he said, describing the IAEA as "fair but firm".