The UN atomic watchdog on Tuesday said it had credible information Iran was working on developing nuclear weapons, its toughest-talking assessment yet which Tehran rejected as baseless.
The International Atomic Energy Agency said it had "serious concerns" based on "credible" information indicating that the Islamic republic "has carried out activities relevant to the development of a nuclear explosive device."
The United States said the report showed that Iran had lied and said it would seek to ratchet up pressure and may seek new sanctions, but analysts said the report would not be enough to get China and Russia on board for such a move.
Although some of the activities listed in 12 dense pages of intelligence "have civilian as well as military applications," the keenly-awaited report said that "others are specific to nuclear weapons."
Iran's envoy to the Vienna-based IAEA, Ali Asghar Soltanieh, told the Iranian news agency Fars that the report was "a repetition of old claims which were proven baseless by Iran in a precise 117-page response."
Using input from more than 10 foreign intelligence agencies plus its own information, the IAEA report listed in considerable detail Iranian work in 12 areas that practically covered every area needed for a weapon.
The picture is "pretty comprehensive when you want to develop a nuclear weapon. It has the core itself, it has a delivery system, it has the acquisition of the material," a senior official familiar with the IAEA probe said.
These included computer modelling of a nuclear warhead; testing explosives in a large metal chamber at the sprawling Parchin military base near Tehran; and studying how to arm a Shahab 3 medium-range missile with an atomic warhead.
"If you put all of this information together it paints a picture that pretty clearly shows that they had nuclear weapons in mind," nonproliferation analyst Peter Crail from the Washington-based Arms Control Association told AFP.
"Iran is on very shaky ground trying to defend a lot of these activities."
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The IAEA, whose board could decide to report Tehran to the UN Security Council again next week, called on Iran "to engage substantively with the agency without delay for the purpose of providing clarifications."
US Senator John Kerry, head of the Senate Foreign Relations Committee, said the report made it clear that Iran "has not been truthful" and that the international community had to "increase pressure" on Tehran.
But Iranian Foreign Minister Ali Akbar Salehi, speaking in Armenia before the report was released, said there was "no serious proof that Iran is going to create a nuclear warhead."
"We have repeatedly stated that we are not going to create nuclear weapons," Salehi said. "Our position has always been that we will never use our nuclear programme for purposes other than peaceful ones."
President Mahmoud Ahmadinejad said his country "does not need an atomic bomb" and would instead "act thoughtfully" to confront US threats against it, according to state media.
However he warned: "If America wants to confront the Iranian nation, it will certainly regret the Iranian nation's response."
The hawkish foreign minister of Iran's arch-foe Israel, Avigdor Lieberman, said before the release of the report that only "crippling sanctions" would be able to thwart Iran, the Maariv newspaper reported.
Russia meanwhile expressed anger over the publication of the report, saying it risked damaging the chance of a renewal of nuclear talks, saying it was "gravely disappointed and bewildered."
Earlier Russian President Dmitry Medvedev, speaking in Berlin, said that Israeli threats were "extremely dangerous rhetoric" that could result in a "catastrophe" for the Middle East.
Germany's foreign ministry called for "greater political and diplomatic pressure" on Iran, while French Foreign Minister Alain Juppe said sanctions should be toughened but "everything must be done" to avoid a military conflict.