Iran's nuclear programme is “galloping ahead” and the International Atomic Energy Agency has very limited visibility on what is happening, IAEA chief Rafael Grossi told Spain's El Pais newspaper in an interview published on Friday.

In June, Iran began removing essentially all the agency's monitoring equipment, installed under its 2015 nuclear deal with world powers.

Grossi said at the time it could be a “fatal blow” to chances of reviving the deal following 2018's pullout by the United States.

“The bottom line is that for almost five weeks I have had very limited visibility, with a nuclear programme that is galloping ahead and, therefore, if there is an agreement, it is going to be very difficult for me to reconstruct the puzzle of this whole period of forced blindness,” he told El Pais.

Grossi had said in June there was a window of just three to four weeks to restore at least some of the monitoring that was being scrapped before the IAEA lost the ability to piece together Iran's most important nuclear activities.

Since then-US president Donald Trump pulled Washington out of the deal and re-imposed sanctions against Tehran in 2018, Iran has breached many of the deal's limits on its nuclear activities.

It is enriching uranium to close to weapons-grade.

Read: Iran ‘capable of making N-bomb’, says Ayatollah Khamenei’s aide

Western powers warn it is getting closer to being able to sprint towards making a nuclear bomb. Iran denies wanting to.

“It is not impossible (to reconstruct the puzzle), but it is going to require a very complex task and perhaps some specific agreements,” Grossi, who was visiting Madrid, said in his interview with El Pais.

Indirect talks between Iran and the United States on reviving the 2015 deal have been stalled since March.

Grossi said he was concerned and worried about these weeks with no visibility.

“The agency needed to reconstruct a database, without which any agreement will rest on a very fragile basis, because if we don't know what's there, how can we determine how much material to export, how many centrifuges to leave unused?” he said.

Asked about a Reuters report that Iran is escalating its uranium enrichment further with the use of advanced machines at its underground Fordow plant, Grossi said “the technical progress of the Iranian programme is steady”.