Such an attack "might have an amalgamating effect which would make a determination on the part of Iran to go to a nuclear weapon or to abandon the treaty on non-proliferation. I’m telling you this because they have told me,” Grossi said.
The IAEA chief emphasized the difficulty any military operation would have of destroying Iran's nuclear capabilities.
“Certainly this program runs wide and deep. And when I say deep, I know what I’m saying. So many of these facilities are extremely well protected. This would require a very, very devastating force to affect it.”
The United States and Iran are locked in high-stakes negotiations over Tehran's nuclear program, and US President Donald Trump has mooted an attack if they fail.
Israel struck multiple targets inside Iran in October and has repeatedly vowed to deny Iran a nuclear weapon, which it views as an existential threat.
Tehran denies seeking nuclear weapons but its arch-nemesis Israel and Western countries doubt its intentions.
Grossi said Iran needed to the let the nuclear watchdog do its job by allowing more inspection and explaining past behavior.
"I am in a constant dialog, especially with the foreign minister (Abbas) Araghchi," Grossi said. "(They say) they have nothing happened. If this is the case, then they should allow us to do our job and to get as much access as we can."
Western powers may push for a resolution finding Tehran non-compliant with its nuclear non-proliferation obligations at the upcoming quarterly meeting of the IAEA Board of Governors this week.
Iran has warned of an unspecified strong response to any such move and has accused the IAEA of being swayed by its Western adversaries.
The United States accused Iran of seeking a nuclear weapon as part of the shadowy Amad Project scrapped in 2004, an effort Washington says was overseen by Iran’s Organization of Defensive Innovation and Research.
"There are areas of real concern and we need to get to the bottom of a number of things," Grossi said, without referring to any specific issues.
"Over the past few years ... we discovered traces of enriched uranium in places that were not supposed to be having any nuclear activity. So my question here - this is not an accusation - It's just a very logical question: if we found these traces there, what was going on?"
The IAEA would likely play a central role in verifying and overseeing any nuclear agreement between Iran and the United States, as it did with an international deal US President Donald Trump scrapped in 2018.
Hawks in Israel and the United States have said Iran's lack of full disclosure on past activities calls into question its commitment to any future deal.
"For years ... we have been getting some answers, and these answers have not been satisfactory," Grossi added. "Please do not believe that we are naive."
