The Director General of the International Atomic Energy Agency (IAEA) Raphael Grossi said any US and Israeli attack on Iran would be "illegal".
"Any attack on Iran's nuclear sites would violate international law whether by Israel or any other country," Grossi said.
The IAEA said on Saturday that Grossi had received sweeping assurances from Iran that it will assist a long-stalled investigation into uranium particles found at undeclared sites and re-install removed monitoring equipment.
Grossi was in Tehran last weekend and met with the head of the Iranian Atomic Energy Organisation, Mohammad Eslami, and President Ibrahim Raisi.
A joint statement issued by the Atomic Energy Organisation of Iran and the IAEA said the talks focused on "the importance of taking steps for broader cooperation in order to expedite the resolution of remaining safeguards issues in an appropriate manner."