"If Iran refuses to abide by the demands of the international community and rejects all serious cooperation, we are prepared to adopt, with those countries that follow, unprecedented sanctions," the French foreign ministry said in a statement.
British Foreign Minister William Hague also warned of “additional measures” against Tehran.
Activities outlined in the report “have no credible civil purpose", Paris said, adding that it “leaves little doubt about Iran's intentions".
The International Atomic Energy Agency (IAEA) expressed "serious concerns regarding
possible military dimensions to Iran's nuclear programme" in a 1,000-page report based on its own research and information provided by various countries’ intelligence agencies.
“This information indicates that Iran has carried out activities relevant to the development of a nuclear explosive device," it said.
Although its findings agreed with a 2007 US intelligence report suggesting Iran had scrapped an official nuclear weapons drive in 2003, the IAEA suspects some covert work had continued.
The IAEA report includes charges of:
- computer modelling of a nuclear warhead;
- testing explosives in a large metal chamber at the sprawling Parchin military base near Tehran;
- studying how to arm a Shahab 3 medium-range missile with an atomic warhead.
Iran denied the claims and President Mahmoud Ahmadinejad said his country “will not budge an iota” from its nuclear plans.
And deputy armed forces chief, Brigadier General Masoud Jazayeri, warned that any sign of Israel carrying out threats to attack Iran's nuclear sites "will see its destruction".
Israeli media concluded that the report made an Israeli attack, which has been openly discussed over the past few days, less likely because it increased the chances of tougher international sanctions.
But Deputy Foreign Minister Gennady Gatilov declared any such move “unacceptable”.
"Any additional sanctions against Iran will be interpreted by the international community as a means of changing the regime in Tehran," he told the Interfax news agency.