Iran's nuclear programme is "galloping ahead" with very little outside scrutiny of what is happening, the head of the International Atomic Energy Agency, Rafael Grossi, told Spain's El Pais newspaper on Friday.
In June, Iran began removing all the agency's monitoring equipment, installed under its 2015 nuclear deal with world powers.
Mr Grossi said at the time this could deal a "fatal blow" to chances of reviving the deal following the 2018 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.
"It is not impossible, but it is going to require a very complex task and perhaps some specific agreements," Mr Grossi said during a visit to Madrid.
He 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.
Iran has breached many of the deal's limits on its nuclear activities since then-US president Donald Trump pulled Washington out of the agreement and re-imposed sanctions on Tehran in 2018. It is enriching uranium to close to weapons-grade levels.
Western powers say Iran is getting closer to being able to sprint towards making a nuclear bomb. Iran denies seeking nuclear weapons.
Indirect talks between Iran and the United States on reviving the 2015 deal have been stalled since March.
Iran's Foreign Minister Hossein Amirabdollahian was quoted on Friday as saying his country and the US were very close to a deal to revive the 2015 accord, but that Tehran needed guarantees to avoid getting "bitten twice".
"We have a ready text in front of us and we agree on more than 95 to 96 per cent of its content, but there's still an important flaw in this text: we need to get the full economic benefits of the agreement. We don't want to be bitten twice," Iranian media quoted Mr Amirabdollahian as saying.
US officials have warned that if Iran fails to agree to a deal then more sanctions and other curbs would be announced.
The US and Israel signed a pledge to prevent Iran from developing nuclear weapons, including by force if needed, during President Joe Biden's visit to the region this month.
Mr Biden later attended a summit of GCC states, Iraq, Egypt and Jordan in the Saudi city of Jeddah where leaders issues a joint statement calling on Iran to co-operate with the IAEA.
Mr Grossi said he was concerned and worried about the 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 was escalating its uranium enrichment further with the use of advanced machines at its underground Fordow plant, Mr Grossi said "the technical progress of the Iranian programme is steady".
With reporting from Reuters.