A UN team was allowed access to inspect the Qom site last month
The UN's nuclear watchdog says it needs "more clarification" about the purpose of a recently declared Iranian nuclear site near the city of Qom.
The International Atomic Energy Agency also said in a new report the delayed declaration of the plant raised concern about other possible secret sites.
A UN team was allowed access to inspect the Qom site last month.
Some Western nations fear Tehran is trying to build nuclear weapons but Iran says its programme is peaceful.
Chief Iranian nuclear negotiator Ali Asghar Soltaniyeh told Iranian media that Tehran had "provided all the information about the new facility" and called the IAEA report "repetitive".
The US said the report showed Iran had not met its international obligations.
Earlier, Russia said a nuclear power station it had been building at Bushehr in southern Iran would not be completed by the end of this year as planned.
The IAEA report said it expected the Qom site to start enriching uranium in 2011.
Paul Reynolds, BBC world affairs correspondent
The IAEA report on the site near Qom basically confirms that it will be capable of housing some 3,000 centrifuges for uranium enrichment. Iran has said that it is building the plant to protect its technology from potential attack, and the head of the IAEA has said there is nothing to worry about at the site.
But what does worry the governments trying to negotiate with Iran is the secrecy that surrounded the plant. The report says such secrecy does not "contribute to the building of confidence".
In the wider picture, there is still stalemate over the proposal to take Iran's stockpile of low-enriched uranium and enrich it in Russia and France. Senior diplomats from the US, Russia, China and the EU will meet soon as pressure grows for further sanctions on Iran if no agreement on its uranium enrichment can be reached by the end of the year.
It said: "Iran's explanation about the purpose of the facility and the chronology of its design and construction requires further clarification."
Iran only revealed the existence of the Fordo enrichment facility, which is being built about 30km (20 miles) north of Qom, in September.
The IAEA report said this did "not contribute to the building of confidence" and "gives rise to questions about whether there were any other nuclear facilities not declared to the agency".
Iran's delay in notification was "inconsistent with its obligations", the report said.
The BBC's Iran correspondent Jon Leyne says the report raises key questions about the timing of the site's construction.
He says Iran's declaration that it began the project in 2007 does not square with the IAEA's evidence that there was work there as far back as 2002.
The report said satellite imagery showed there was work in Qom between 2002 and 2004, and that this had resumed in 2006 and "continued to date".
Mr Soltaniyeh said he was "comfortable" with the report, as it confirmed Iran was "fully co-operating" and that the activities at Qom were "in accordance with the IAEA instructions and limitations".
Russia has delayed the launch of the Bushehr plant
He told al-Alam TV: "Iran has provided all information about the new facility and the material inside it.
"We will later proceed with installing the required equipment. The facility will go online in 2011.
"Inspectors scoured the facility for two complete days. Everything was compatible with the non-proliferation treaty."
But the US state department said Iran was still refusing to comply fully with its international nuclear obligations.
Spokesman Ian Kelly said: "Now is the time for Iran to signal that it wants to be a responsible member of the international community."
On Sunday, Russia and the US both warned Iran that time was running out for talks over its nuclear programme.
NUCLEAR FUEL CYCLE
Mined uranium ore is purified and reconstituted into solid form known as yellowcake
Yellowcake is converted into a gas by heating it to about 64C (147F)
Gas is fed through centrifuges, where its isotopes separate and process is repeated until uranium is enriched
Low-level enriched uranium is used for nuclear fuel
Highly enriched uranium can be used in nuclear weapons
Moscow on Monday said the delay in launching the Bushehr plant was for "technical reasons".
Russian officials had said earlier this year that the plant would be completed before the end of 2010, but Russian Energy Minister Sergei Shmatko said on Monday that although progress had been made, there would be no launch.
The BBC's Richard Galpin in Moscow says the decision to delay the completion is clearly political - an expression of Russia's frustration at Iran's failure to accept an offer now on the table from the international community.
Under a plan brokered by the IAEA and agreed by Russia, the US and France, Iran would send about 1,200kg (2,600lb), or 70%, of its low-enriched uranium, to Russia by the year's end for processing.
Subsequently, France would convert the uranium into fuel rods for use in a reactor in Tehran that produces medical isotopes.
This is seen as a way for Iran to get the fuel it needs, while giving guarantees to the West that it will not be used for nuclear weapons.
Iran has raised "technical and economic considerations" with the IAEA and has missed deadlines to respond.
US President Barack Obama has said it is unfortunate that Iran seems unable to say yes to a "creative" international plan to allay suspicions that it is secretly developing nuclear weapons.