A senior Iranian official has denied allegations that Tehran is hiding a nuclear bomb facility.
Iran's enrichment activities are under international scrutiny
The National Council for Resistance in Iran - an exiled opposition group - said a secret uranium enrichment site had been built just north of Tehran.
But Hossein Moussavian, one of Iran's top negotiators, said the facility had nothing to do with nuclear activities.
Iran also said that four people have gone on trial in Tehran for allegedly spying on nuclear facilities.
The NCRI - an exiled opposition group - said on Wednesday that a secret uranium enrichment site had been built just north of Tehran.
The NCRI is the political wing of the People's Mujahideen Organisation. Both are banned as terrorist groups in the US and EU.
There was no confirmation of the identity of the suspects sent to trial, but Iranian intelligence minister Ali Yunessi said in August that a number of people arrested on nuclear spying charges were linked to the two opposition groups.
The group first revealed the existence of two key nuclear sites in Iran four years ago, prompting international inspections.
This time it alleged that the former head of Pakistan's nuclear programme, Abdul Qadeer Khan, had given Iran designs for a nuclear bomb, as well as highly enriched uranium.
Many in Tehran saw these allegations as deliberately timed to try to scupper a deal over Iran's nuclear programme with Europe, reached days ago.
But US Secretary of State Colin Powell said he could corroborate some of the charges made by Iranian dissidents based on intelligence he had received.
Mr Powell said intelligence showed Iran had been actively working on adapting its missiles to carry nuclear warheads.