The UN atomic watchdog on Tuesday released its toughest-talking assessment yet on Iran's suspected nuclear weapons drive, based on a large body of intelligence rejected in advance by Tehran as fabricated.

In a keenly awaited report seen by AFP, the International Atomic Energy Agency expressed "serious concerns" and said some of the activities listed in 12 dense pages of intelligence "have civilian as well as military applications."

But it stressed that "others are specific to nuclear weapons," saying it had overall "credible" information indicating the Islamic republic "has carried out activities relevant to the development of a nuclear explosive device."

Iran, which diplomats said had seen an advance copy of the report, insisted Tuesday the West had no proof it was developing nuclear weapons, with President Mahmoud Ahmadinejad declaring that Tehran did not even need the bomb.

TV channels in Israel, which has stoked speculation in recent days of a pre-emptive strike on Iran, said the report's "unprecedented severity" made an imminent Israeli military strike unlikely.

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.

"Iran's leaders know what they need to do, the question is how we ensure they start doing it," Kerry said.

A senior official familiar with the IAEA probe said that the activities listed in the report covered practically all areas needed to make the bomb.

The picture painted 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," the official said.

"The report details Iranian activities that can only be explained if the purpose was to develop a nuclear bomb," one Western diplomat in Vienna said, adding that Tehran had "offered no plausible explanation."

The agency said that its information indicated that "prior to the end of 2003, these activities took place under a structured programme, and that some activities may still be ongoing."

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."

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 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."

Avigdor Lieberman, Israel's hawkish foreign minister, 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 between Tehran and the international community.

"Russia is gravely disappointed and bewildered that the report is being turned into a source adding to the tensions over the problems connected to the Iranian nuclear programme," the foreign ministry in Moscow said.

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.