The US has dismissed a tour by journalists of a nuclear facility in Iran as a "staged media event".
Iran insists Natanz is helping to develop nuclear energy, not bombs
The state department said Iran should be answering questions about what it is doing at the Natanz site, the existence of which was kept secret until 2002.
Journalists accompanied Iran's president to the site on Wednesday.
The US is calling for Iran to give international inspectors full access to its nuclear facilities to allay fears that it has a secret arms programme.
Tehran says its nuclear programme is for peaceful, civilian use only.
But US Deputy State Department spokesman Adam Ereli said: "If Iran were really serious about demonstrating transparency in its nuclear program, it should answer all of the International Atomic Energy Agency's outstanding questions.
"The point here is that if there is a commitment to transparency, there are real effective, meaningful ways to demonstrate that commitment, beyond a staged media event like is being reported."
He said Iran should allow give inspectors from the IAEA "full and unrestricted access to suspicious sites", and allow its senior nuclear officials to be interviewed.
Tehran allowed more than 30 local and foreign journalists to accompany Iranian President Mohammed Khatami on his official visit to Natanz, about 250km (150 miles) south of Tehran.
They were taken deep inside a building to a vast empty hall designed to house 50,000 enrichment centrifuges, Reuters news agency reported.
The enrichment facility was built more than 18m (54ft) below ground because of "security problems", Iranian officials were quoted as saying.
But defence experts say it could be a precaution against possible aerial attacks from the US and Israel.
Dozens of anti-aircraft placements were also spotted by journalists on the approach to Natanz.
Mohammad Saeedi, deputy head of Iran's Atomic Energy Organisation, said the site was being inspected by the IAEA.
"IAEA inspectors visit this facility at least once a month and also use a monitoring system to check the suspension," he said.
But he said staff were keen to resume enriching uranium, which was frozen while Iran negotiates with the EU over its nuclear programme.
"The people involved in the project are frustrated by the suspension," he said. "They hope there is an agreement with the Europeans so that all activities can be resumed. Enrichment is Iran's right."