France has for the first time explicitly accused Iran of using its nuclear programme as a cover for clandestine military nuclear activity.
Iran insists the programme is for civilian purposes
Foreign Minister Philippe Douste-Blazy told French TV no civilian programme could explain Iran's activity.
Iran says it resumed small-scale uranium enrichment work last week, after the UN nuclear watchdog reported it to the Security Council.
But Tehran insists the programme is solely for peaceful purposes.
Iran's chief nuclear negotiator sharply denied the charges made by Mr Douste-Blazy.
"Contrary to all the propaganda against us, we are not seeking a nuclear bomb, since we are a signatory to (the nuclear) Non-Proliferation Treaty (NPT)," he said on French radio.
"We are a responsible country - it is Western propaganda that keeps on saying that Iran is seeking a bomb, but it is not true."
BBC diplomatic correspondent Jonathan Marcus says that Mr Douste-Blazy's blunt characterisation shows that, in diplomatic terms at least, the gloves are coming off.
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France, the UK and Germany have had a key role in pursuing long-running contact with Tehran, in an effort to persuade it to give up its plans.
But the mood among the Europeans is sombre, laced with an element of frustration, our correspondent says - as Iran now appears intent on pursuing its nuclear research programme.
Meanwhile, China has expressed concern about the nuclear issue.
Foreign Ministry spokesman Qin Gang said Beijing wanted a peaceful solution by diplomatic means.
The escalation of the crisis has come as US Secretary of State Condoleezza Rice asked Congress for an additional $75m to subsidise dissident groups and to fund a 24-hour television station broadcasting in Farsi.
The International Atomic Energy Agency resolution - supported by all five permanent members of the UN Security Council - could lead to eventual sanctions against Iran, although any action has been put off until a report by the head of the agency on 6 March.
Iranian officials are due to hold talks with Russia on 20 February to discuss uranium enrichment on Russian soil as a possible compromise.
But Mr Douste-Blazy said Iran was being disingenuous.
"No civil nuclear programme can explain the Iranian nuclear programme. It is a clandestine military nuclear programme," he said.
"The international community has sent a very strong message to the Iranians - show reason, suspend all nuclear activities and uranium enrichment. And they're not listening to us."
"That is the reason why, for the first time for days, the international community is united. It's not just the Europeans - France, Germany and the British - it's also Russia and China."