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Yukiya Amano, the head of the International Atomic Energy Agency, was speaking at the body's headquarters as its board of governors began a routine meeting just weeks after I.A.E.A. inspectors returned from talks in Tehran that failed to obtain access to the Parchin site, 20 miles south of Tehran.

"I request Iran once again to provide access to the Parchin site without further delay," Mr. Amano said. "Providing access to the Parchin site would be a positive step which would help to demonstrate Iran's willingness to engage with the agency on the substance of our concerns."

The talks about Parchin are separate from the negotiations Iran is conducting with six global powers on the broader question of its disputed nuclear program, which Tehran says is for peaceful purposes. Western powers suspect that Iran is seeking the technology for nuclear weapons.

The last round of those talks ended last week in Almaty, Kazakhstan, with agreement for further meetings in March and April over a proposal that would sharply constrain Iran's stockpile of the most dangerous enriched uranium in return for a modest lifting of some sanctions.

The six powers dropped their demand that Iran shut down its enrichment plant at Fordo, built deep underneath a mountain, instead insisting that Iran suspend enrichment work there and agree to take a series of steps that would make it hard to resume producing nuclear fuel quickly.

The six also agreed, in another apparent softening, that Iran could keep a small amount of 20 percent enriched uranium — which can be converted to bomb grade with modest additional processing — for use in a reactor to produce medical isotopes.

Mr. Amano said on Monday that "Iran is not providing the necessary cooperation to enable us to provide credible assurance about the absence of undeclared nuclear material and activities. The agency therefore cannot conclude that all nuclear material in Iran is in peaceful activities."

He recalled that Iran has begun installing more sophisticated centrifuges at its principle enrichment plant at Natanz.

"We must not lose sight of the ultimate goal, which is to resolve all outstanding issues related to Iran's nuclear program," Mr. Amano said. "Dialogue should produce results."