WASHINGTON — Senior US Senator John Kerry on Tuesday urged sustained 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," 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.

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."

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.

"Clearly the United Nations, not known for speaking clearly on many topics, is now telling us one clear thing -- that the Islamic Republic of Iran is designing and moving towards building nuclear weapons," he said.