Ali Akbar Salehi, director of the Iranian Atomic Energy Agency, called the new facility at the Qom site "very small" and "a back-up installation," the official Iranian news agency IRNA reported Sunday.
Salehi repeated earlier assertions that Iran complied with all international regulations in reporting the existence of the planned site, according to the IRNA report.
Two U.S. administration officials told CNN on Sunday that the United States plans to tell Iran this week it must provide "unfettered access" to the Qom site, the people involved in its construction and the timeline of its construction "within weeks."
The latest dispute comes ahead of planned talks Thursday involving Iran and the United States, Britain, Germany, France, Russia, and China about international concern over Iran's nuclear ambitions.
In interviews broadcast Sunday, top U.S. officials said Iran's newly revealed underground nuclear facility violates international requirements for reporting such operations, reinforcing the perception that Iran is trying to hide a weapons program.
"If they wanted it for peaceful nuclear purposes, there's no reason to put it so deep underground, no reason to be deceptive about it, keep it a ... secret for a protracted period of time," Gates said in the interview recorded Friday.
In a separate interview on the CBS program "Face the Nation," Secretary of State Hillary Clinton called for the strongest possible sanctions if Iran can't prove a peaceful intent for the newly disclosed facility and its entire nuclear program.
"It would have been disclosed if it were for peaceful purposes," said Clinton, who also was interviewed Friday. She added that Iran must do more than provide assurances at the meeting on Thursday, because past assurances proved false.
"They can open up their entire system to the kind of extensive investigation that the facts call for," Clinton said. Later, she said: "The Iranians keep insisting no, no, that's for peaceful purposes. That's fine. Prove it. Don't assert it. Prove it."
Clinton acknowledged that the United States knew of the previously undisclosed Iranian enrichment plant before Iran reported its existence to the IAEA.
After the interview with Clinton took place on Friday, Iran announced it would allow international inspection of the plant and said it met international guidelines for disclosing such a facility. Iran also repeated its insistence that its nuclear program is for peaceful energy production.