The International Atomic Energy Agency director general told the Munich Security Conference that inspectors had returned to Iran after attacks by Israeli and US forces last year but had not been able to visit any of the sites targeted.
The United States has positioned one aircraft carrier strike group in the Middle East, and US media say a second group could arrive within two weeks amid reports that US President Donald Trump is considering new military action.
Grossi said the dialogue with Iran since the inspectors' return last year had been "imperfect and complicated and extremely difficult, but it's there".
He said an accord with Iran on returning to the damaged sites was "absolutely possible" but would be "terribly difficult".
"The big question of the moment is how to define these steps for the future. And we know perfectly well what needs to be checked and how to check it."
The US has had contact with Iranians officials in recent weeks. But Iran has refused access to the key nuclear sites that came under attack when Israel launched a 12-day war last June, followed by the US attacks. It insisted in November that visits had to be part of a new accord.
Washington has said it wants to pursue talks with Iranian officials after a first meeting in Oman on February 6.
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