Top US lawmakers called Tuesday for renewed sanctions pressure on Iran after the UN nuclear watchdog agency released a report seen as backing Western charges that Tehran seeks atomic weapons.

"The facts of the IAEA report make it clear that Iran has not been truthful about its nuclear program," Senator John Kerry, a Democrat who chairs the Senate Foreign Relations Committee, said in a statement.

"The international community must continue to stand strong and increase pressure on Iran to comply with its international obligations. Iran's leaders know what they need to do, the question is how we ensure they start doing it."

Tehran has denied charges by Western nations that it seeks nuclear arms under the cover of a civilian energy program and rebuffed demands from world powers to freeze uranium enrichment efforts that can be a key step towards atomic weapons.

But Senate Intelligence Committee Chairwoman Dianne Feinstein called the report "one more step toward what has become an inescapable conclusion: Iran is, in fact, building the capability to produce nuclear weapons."

"Now is the time for Iran to answer these concerns, to abide by its commitments and demonstrate to the world that it is not seeking a nuclear weapon," said Feinstein, a Democrat.

The top Republican on Feinstein's panel, Senator Saxby Chambliss, said the report showed Tehran's activities "cannot be explained away as dual-use nuclear energy technology."

Chambliss called for "renewed urgency in dealing with Iran's unrelenting nuclear ambition" in light of the IAEA findings, alleged Iran-backed attacks on US forces in Iraq and Afghanistan, and an apparently Iranian-supported plot to kill the Saudi ambassador to Washington.

Kerry said he would work closely with President Barack Obama's administration "to take whatever additional necessary and productive steps to get Iran to meet the coalition's demands."

"The international community must step up its support and enforcement of a robust sanctions regime -- or risk an unthinkable alternative. Time is short and options are limited," said Representative Howard Berman, the top Democrat on the House Foreign Affairs Committee.

Berman crafted a measure, approved by the committee last week, that aims to isolate Iran's central bank from the international financial system, eroding Iran's resources for pursuing its nuclear program.

Even as Iran denounced the agency's massive body of intelligence as fabricated, Kerry said he had recently met with International Atomic Energy Agency (IAEA) Director General Yukiya Amano and vouched for his integrity.

"He is a thoughtful man who is interested only in reporting the facts. He has no other agenda," said the senator.

The agency's report, seen by AFP, said it has "serious concerns regarding possible military dimensions to Iran's nuclear program."

It said it had its "credible" information from foreign intelligence reports and its own research that indicates that Iran "has carried out activities relevant to the development of a nuclear explosive device."

It added: "The information also indicates that prior to the end of 2003, these activities took place under a structured program, 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."

House Foreign Affairs Committee Chairwoman Ileana Ros-Lehtinen called the report "further proof that the US and other responsible nations must take decisive action to stop the regime from acquiring a nuclear capability."

Ros-Lehtinen, a Republican, called for the US Congress to "ratchet up the pressure" with new sanctions legislation to "cripple" Iran's ability to pursue nuclear program.

"If the Iranian regime acquires nuclear weapons capabilities, the US, Israel, and our other allies in the region and around the world will face an unimaginable threat to our security. The clock is ticking," she said.

Republican Senator Mark Kirk called for Washington to take steps towards "collapsing the central bank of Iran" in light of the report.

"If this is not a smoking gun, I don't know what is," he said.

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.