The UN atomic watchdog's hardest-hitting report to date on Iran's suspected nuclear weapons drive is probably too weak to convince Russia and China of the need for more sanctions, analysts said Wednesday.

The International Atomic Energy Agency said in a keenly awaited report circulated late Tuesday it had broadly "credible" intelligence suggesting Iran had done work towards building nuclear warheads.

Publishing reams of input from foreign intelligence agencies, and backed up by its own information, the IAEA said the Islamic republic had engaged in activities that could only conceivably have one purpose: producing the bomb.

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.

Significantly, although its findings concurred with a much-cited 2007 US intelligence report suggesting Iran had scrapped an official nuclear weapons drive in 2003, the IAEA also suspects some covert work had continued.

The United States said late Tuesday it would ratchet up pressure and consult its partners on possible new sanctions, while fellow permanent UN Security Council member France called Wednesday for "unprecedented sanctions."

"The overwhelming impression is that over 20-25 years Iran ... consistently engaged in activities that are useful for nuclear weapons production," Mark Hibbs from the Carnegie Endowment for International Peace think-tank told AFP.

"Some of the activities may have another use... but it is the context which is important. We are talking about a country that has been hiding its activities and that has been deceiving the agency for a period of over two decades."

Peter Crail from the Arms Control Association in Washington agreed, but he said that the report did not constitute a "smoking gun" proving conclusively that the Iranians "are on the verge of making a nuclear weapon."

"This isn't new blockbuster information," Crail told AFP.

"It is just more detail behind the current assessments, that Iran has been trying to build different aspects of a nuclear weapons programme but that it hasn't yet made a decision to put all those together and actually pursue a bomb."

The Institute for Science and International Security (ISIS) said that if it were true that Iran engaged in "weaponisation activities" after 2003, this would constitute a "major violation" of the nuclear non-proliferation treaty (NPT).

"Notably absent, however, is any assessment by the IAEA of Iran's capability to make a nuclear explosive device based on what is learned through these activities," the Washington-based think-tank said in a report.

As a result, analysts said it would be tough to convince Russia and China of the need for the IAEA's board to pass a resolution when it meets next Thursday and Friday reporting Iran to the UN Security Council again for non-compliance.

Such a resolution, even if passed by the board where a simple majority is needed, would in any case be dead in the water by the time it arrived in New York, where Moscow and Beijing both wield a veto.

"It is unlikely, particularly given the position of Russia and China, that there will be additional sanctions," Crail said.

Hibbs agreed: "Six weeks ago we were hearing from these countries (China and Russia) that they were not willing to support that resolution. That resolve on the Russian and Chinese side appears to have remained firm."

Moscow on Tuesday even went so far as to express anger over the publication of the report, saying it risked damaging the chance of a renewal of talks, with the foreign ministry saying it was "gravely disappointed and bewildered."

But Crail added the report was damning enough to build a "very strong case" for strengthening the existing sanctions, saying countries still had a "lot of latitude" to impose more restrictions within the existing legal framework.

Iran worked on developing nuclear weapons: UN report
Vienna (AFP) Nov 8, 2011 - 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."

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.