A top Iranian official told the UN that Iran was not willing to submit to tougher inspections of its facilities.
The US has accused Iran, which is building a centrifuge plant at Natanz in southern Iran, of having secret plans to make nuclear weapons. It fears Iran could enrich weapons-grade uranium at the site.
Iran's atomic energy chief Gholamreza Aghazadeh told the International Atomic Energy Agency on Tuesday that Iran's nuclear program was "only for peaceful purposes," a diplomat who attended the meeting said.
Aghazadeh told the UN nuclear watchdog group at the closed door meeting that Iran need the facilities to make its own nuclear fuel, according to the diplomat.
Iranian officials have said they have nothing to hide because their nuclear program is only meant to generate electricity.
But in Washington, US State Department spokesman Richard Boucher said the evidence collected by the UN agency and even Iranian statements pointing to a weapons program.
"Despite their protests, despite their claims, Iran is developing a full-scale nuclear program that it would not behoove anybody to cooperate with," Boucher said.
"And so we will keep making the case. We will keep making the point with the information that is available, and I would say increasingly available, that Iran's nuclear ambitions are much bigger than many had hoped," Boucher added.
In Moscow, meanwhile, Deputy Foreign Minister Alexander Losyukova said there was no evidence Iran was pursuing a nuclear weapons.
"Very sound evidence is needed to accuse anyone. So far, neither the United States nor any other countries can present it," Losyukov said Tuesday, according to the Interfax news agency.
Losyukov acknowledged that Iran's nuclear program had some uncertainties, and that Moscow would work with Tehran to "add more transparency" to its program. As for Russian-Iranian nuclear cooperation, Losyukov said the work was "strictly in line with IAEA norms."
Russia's nuclear cooperation with Iran has long been a contentious issue between Washington and Moscow. The US claims that the technology and expertise Iran is gaining from Russia's construction of the US$800 million Bushehr nuclear power plant could be used for a weapons program, and that Russian companies -- perhaps without official permission -- have transferred weapons technology to Tehran.
Aghazadeh also faced "tough questioning" from the representatives of 10 countries, including the US, Britain, Japan, Canada, Netherlands and France, the diplomat said.
The main controversy centered on the UN agency's request that Iran agree to more intrusive inspections, the diplomat said. Washington has also urged Iran to agree to the tougher regime.
Aghazadeh, however, said it would "depend on conditions. ... It was very conditional," the diplomat said.
The IAEA oversees compliance with the Nuclear Nonproliferation Treaty, which Iran has signed. The head of the Vienna-based agency visited Iranian nuclear sites in February and is expected to report to the agency's board in June.
Washington claims that Iran has tested nuclear material at the site without declaring it to the UN agency, and is pushing the agency to declare Iran in noncompliance with its safeguards agreement.
IAEA spokeswoman Melissa Fleming said that "inspections and analysis" of Iran's nuclear sites were still under way, and that the report to be delivered in June was not yet ready.