His comments came a day before international talks with Iran in Geneva amid growing concern about the covert build-up of its nuclear programme, with world leaders seeking guarantees that Tehran's nuclear plans are peaceful.
"Iran has been on the wrong side of the law in so far as the IAEA regulation to inform the agency at an earlier date," Mohamed ElBaradei, director of the UN's International Atomic Energy Agency, told Indian TV channel CNN-IBN.
"Iran was supposed to inform us on the day it was decided to construct the facility. They have not done that," ElBaradei said in a transcript on the channel's website.
European Union foreign affairs chief Javier Solana will conduct the Geneva talks with the Iranian nuclear negotiator Saeed Jalili, along with senior officials from Britain, China, France, Germany, Russia and the United States.
Iran's atomic energy chief Ali Akbar Salehi has said he is ready to discuss concerns about its previously undisclosed second enrichment plant, but he has ruled out a freeze on enrichment.
ElBaradei, speaking on a trip to New Delhi, said that Salehi had told him the plant was far from complete and was "just ready in term of cables and construction."
But the IAEA head described Iran's failure to disclose the facility as "a setback to the principle of transparency" and said UN inspectors had to visit the site as soon as possible to assess if it was for peaceful purposes.
Disclosure to the IAEA last week of the plant cast a shadow over the talks in Geneva, with Washington calling on Tehran to agree to "immediate, unfettered access" by IAEA inspectors.
Uranium enrichment is a sensitive issue as the process can produce the fuel for nuclear power or, in highly refined form, the fissile core of an atomic bomb.
The UN Security Council has imposed three sets of sanctions against Iran over its failure to heed repeated ultimatums to suspend enrichment.
The United States and its allies are reportedly considering much tougher sanctions in case of continued Iranian defiance.
Iran has said it is building the plant, located near the holy city of Qom, because of the military threat hanging over its existing nuclear facilities.
Both Washington and Iran's regional arch-foe Israel have refused to rule out military action.
ElBaradei said he hoped the talks starting on Thursday would "usher in a comprehensive, meaningful dialogue."
But Iranian President Mahmoud Ahmadinejad lashed out at world leaders for demanding access to the Qom plant.
"The leaders of these countries made a historic mistake with their comments about the new plant," state television quoted Ahmadinejad as saying on Wednesday.
He said the talks were an "exceptional opportunity for US and a few European countries to correct the way they interact with other world nations."
Western powers suspect the programme is aimed at making atomic weapons, something Iran denies.
