VIENNA, Austria - The UN nuclear watchdog said Friday that Iran is defying a UN Security Council ban on uranium enrichment and accusing the U.S. and its allies of fabricating information to back up claims that Tehran is making nuclear weapons.

At the same time, Tehran has cooperated in other areas of an International Atomic Energy Agency probe, leading the agency to put to rest for now suspicions that several past experiments and activities were linked to a weapons program, said an agency report.

Specifically, the 11-page report obtained by The Associated Press suggested the agency was satisfied with answers provided by Iran on the origin of traces of enriched uranium in a military facility; on experiments with polonium, which can also be used in a weapons program; and on purchases on the nuclear black market.

It said that in those areas information given by Tehran is either "consistent with its findings (or) ... not inconsistent with its findings," suggesting it was content for now with explanations that these activities were not weapons-related.

Still, it said Iran "has not suspended its enrichment-related activities," despite two sets of UN Security Council sanctions over fears the program might be used to make weapons-grade uranium instead of the nuclear fuel Iran says it is interested in.

Instead, said the report, Iran "started the development of new-generation centrifuges" -- an expansion of enrichment -- and continued working on heavy water nuclear facilities. When finished, Iran could cull them for plutonium, a possible fissile payload in nuclear warheads.

Those two findings alone empower the Security Council to slap additional sanctions on Tehran, something U.S officials say is imminent.

The Iranian mission to the United Nations in New York described the findings of the report as "unambiguously attesting to the exclusively peaceful nature of the nuclear program of the Islamic Republic of Iran, both in the past and at present."

But Gregory L. Schulte, the chief U.S. delegate to the IAEA, criticized Iran's failure to explain "detailed activities which ... would be relevant to nuclear weapon research and development."

State Department spokesman Sean McCormack also was dismissive: "We've heard about the Iranians cooperating in the past, yet many questions remain," he told reporters in Washington.

A declassified U.S. intelligence report last December judged that the Iranians had put a nuclear weapons program on hold in 2003. But the U.S., Israel and others contend Iran's continued advances in the crucial centrifuge work will eventually give it a capability to quickly build a bomb.

Much of the information purportedly linking Iran to attempts to make nuclear arms came from the United States, with allies providing lesser amounts and the IAEA passing on selected material to Tehran, after approval by the nations that gave the agency the information.

IAEA chief Mohamed ElBaradei, who drew up the report, said his team had "made quite good progress in clarifying the outstanding issues that had to do with Iran's past nuclear activities, with the exception of one issue, and that is the alleged weaponization studies that supposedly Iran has conducted in the past."

Ahead of the confidential report's release to the 35-nation IAEA board and the UN Security Council, U.S. officials had repeatedly insisted that the IAEA probe would be incomplete unless Iran acknowledged trying to make nuclear arms in the past. That stance is shared by Canada, Japan, Australia and U.S. allies in Europe.

A senior IAEA official, who spoke on condition of anonymity because the report was confidential, said that if the material provided by the U.S. and other agency members on the alleged activities was genuine, most of Iran's work was "most likely for nuclear weapons."

But he said the agency was not reaching any conclusion until the Iranians went beyond rejection of the purported evidence and concretely addressed the issues it raised.

When confronted with some of the documentation from the U.S. and other on its alleged weapons experiments, Tehran "stated that the allegations were baseless and that the information ... was fabricated," the report said.

Iran explained some of its activities linked by the Americans to a weapons program as work on "air bags and for the design of safety belts," according to the report.

The report will be the focus of discussions at an IAEA board report starting March 3. At that meeting, the U.S. and its allies are weighing whether to ask the board to approve a resolution declaring that the agency was unable to shed light on Iran's suspected nuclear weapons program, according to diplomats.