The UN nuclear watchdog has not yet given good enough reasons to visit an Iranian site where it suspects there may have been experiments for developing nuclear weapons, Iranian media said.
The Parchin complex is at the centre of Western suspicions that Iran is developing atom bombs despite Tehran's repeated denials of any such ambition. A report from the International Atomic Energy Agency (IAEA) last week said satellite images showed "extensive activities" at Parchin.
Iranian officials have refused access to the complex, southeast of Tehran, saying it is a military site.
"The reasons and document have still not been presented by the agency to convince us to give permission for this visit," the head
of the Atomic Energy Organization of Iran, Fereydoun Abbasi-Davani, was quoted as saying by Fars news agency on Saturday.
Six world powers failed to convince Iran last week to halt its most sensitive nuclear work, but they will meet again in Moscow next month to try to end a standoff that has raised fears of a new war that could threaten global oil supplies.
Last November, an IAEA report found that Iran had built a large containment vessel in 2000 at Parchin in which to conduct tests that the agency said were "strong indicators of possible weapon development."
In last week's report, the IAEA did not elaborate on what activities it believed were happening there, but Western diplomats suspect Iran is trying to remove any potentially incriminating evidence. Tehran rejects this charge.
After a visit to Tehran last week, IAEA chief Yukiya Amano said he was close to an agreement with Iran on inspection visits to nuclear facilities but some differences remained.
The US-based Institute for Science and International Security thinktank has said there is concern Iran may be trying to cleanse the building at Parchin - possibly by grinding down surfaces, collecting the dust and washing the area thoroughly.
Referring to the Baghdad talks with world powers, Abbasi-Davani dismissed pressure for an end to high-grade uranium enrichment as "predictable".
Iran has begun enriching uranium to 20% fissile purity, saying that is needed for a medical research reactor, but worrying Western countries who see it as a big step towards the 90% purity needed for weapons-grade uranium.
"There is no reason for us to give up enriching uranium to 20% because we produce this fuel only to meet our needs, no
more and no less," the ISNA news agency quoted Abbasi-Davani as saying.
Tehran says it has a sovereign right to enrich uranium, but has sometimes indicated it may be flexible when it comes to higher grade uranium enrichment.
Iran has expanded enrichment at its Fordow nuclear facility, buried deep beneath rock and soil to protect it from air strikes.
Last week's IAEA report said nuclear engineers had installed 50% more enrichment centrifuges at Fordow. Although not yet being fed with uranium, the new machines could be used to further boost Iran's output of uranium enriched to 20%.