"We want to make ourselves available, providing technically sound alternatives to eliminate the possibility that Iran develops a nuclear weapon, to prevent Iran, or to help Iran prove that they don't want to develop a nuclear weapon," Rafael Grossi told reporters at Japan’s National Press Club.
"We hear the government say that. But as somebody said, we trust everybody, but we need to verify. So until we can have a very, very comprehensive watertight system of verification, we will not be satisfied."
Iran denies seeking a nuclear weapon but the United States assesses that it seeks such a capability and Israel sees its arch-foe wanting a bomb to destroy it.
The Atomic Energy Organization of Iran issued a statement shortly after condemning Grossi's remarks as biased and inaccurate, saying "the burden of proof is on the claimant.”
"The IAEA director general, as a senior official of an important international organization, is expected to speak and act impartially, professionally, and without political bias," it said.
"At a time when the United States and certain Western countries are attempting to misuse the IAEA to exert unjust pressure on Iran, such politically motivated and unprofessional statements could serve as a pretext for their illegitimate ambitions.”