Nuclear inspectors monitoring Iran found the country's enrichment program temporarily shut down a week ago, the International Atomic Energy Agency reported Tuesday, reflecting a possible setback for the cornerstone of the country's nuclear activities and source of national pride.

Beyond noting that Iran continued to enrich in defiance of the U.N. Security Council, a report by the U.N. nuclear monitor also said that Tehran for the second year continued to rebuff attempts to investigate suspicions it had experimented with components of a nuclear program.

"Iran has not provided the necessary cooperation to permit the agency to confirm that all nuclear material in Iran is in peaceful activities," said the confidential report obtained by The Associated Press.

A separate report on Syria noted similar stonewalling on the part of Damascus on IAEA attempts to follow up on suspicions that a target bombed by Israeli warplanes more than three years ago was a nearly finished reactor engineered to produce plutonium, and whether the country had hidden other nuclear activities.

The report offered no reason for Iran's enrichment stoppage witnessed Nov. 16 by IAEA staff.

The inspectors were on site at the Natanz enrichment plant in central Iran for only one or two hours, and it was unclear from the report whether the shutdown lasted just hours, days, or longer. A senior diplomat familiar with the agency's overview of Tehran's atomic activities said the Iranians gave IAEA inspectors no time frame or explanation.

Former IAEA Deputy Director General Olli Heinonen, who oversaw the agency's attempts to supervise Natanz and other aspects of IAEA involvement in Iran until his resignation in August, described the shutdown as an "unusual event," adding that during his five-year tenure "we never met this situation."

He also said agency inspectors could estimate how long the shutdown was by calculating missing output from the machines as compared to the amounts they would be expected to produce over a period of time. Those comments suggested the IAEA might be sitting on information on the length of the disruption that it was not willing to share.

Agency officials said there would be no comment beyond what was in the report. But a diplomat familiar with the Iran file said the IAEA likely needed more time and was expected to have more information on the length of the outage in its next report in March.

Heinonen, in a conference call with reporters from the Belfer Center for Science and International Affairs in Cambridge, Massachusetts, suggested any break in enrichment lasting for more than a day indicated a serious problem.

Neither the report nor diplomats who first told the AP of the interruption on Monday, said what could have caused it. But some speculation focused on the Stuxnet worm, the computer virus thought to be aimed at Iran's nuclear program, which experts last week identified as being calibrated to destroy centrifuges by sending them spinning out of control. No one has claimed to be behind Stuxnet, but some analysts have speculated it originated in Israel.

Iran denied that Stuxnet had succeeded in damaging its nuclear program. The country's nuclear chief, Vice President Ali Akbar Salehi, on Tuesday accused the West of being behind what he called a failed sabotage attempt and said details about the virus became known only after Iran's "enemies failed to achieve their goals."

The rare interruption in enrichment is significant against a backdrop of stagnation in the Iranian enrichment effort, appearing to be the latest evidence of serious difficulties in expanding the program after initial rapid growth.

Tehran has taken hundreds of centrifuges off line over the past 18 months, feeding speculation that enrichment was being hampered by major technical issues.

Iran's enrichment program is of international interest because the process can create both nuclear fuel and fissile nuclear warhead material. While Iran insists it wants to enrich only to run a nuclear reactor network, its nuclear secrecy, refusal to accept fuel from abroad and resistance to IAEA efforts to follow up on suspicions of covert experiments with components of a nuclear weapons program have heightened concerns.

Despite four sets of U.N. sanctions Iran insists it will never give up its right to enrich. Since it resumed enrichment four years ago, it has amassed enough low-enriched material for more than two bombs, should it opt to enrich its uranium to weapons-grade levels.

Separately, it has amassed more than 30 kilograms (66 pounds) of higher-enriched uranium, which would take less time to turn into weapons grade material, should Iran decide to do so.

Earlier IAEA reports have documented a drop in operating centrifuges — from 4,920 in May 2009 to 3,772 in September 2010. Tuesday's report showed an increase as of Nov. 5, to 4,816 centrifuges turning out enriched uranium. But the diplomat said that output actually decreased from September to early November, in an apparent reflection of operating problems.

The 9-page report, which was sent to the IAEA's 35 member nations and the U.N. Security Council, mentioned in a brief footnote that "no cascades were being fed" a week ago.

Over the past several months, Iranian officials have acknowledged that the Stuxnet code had spread widely through Iranian industrial sites and infected several personal laptops belonging to employees at the country's first nuclear power plant.

While not confirming or denying the temporary shutdown, Salehi, Iran's nuclear chief, said Tuesday that the malicious computer bug had not harmed the country's atomic program.

"One year and several months ago, Westerners sent a virus to (our) country's nuclear sites," Salehi said, according to the official IRNA news agency. He did not specify which sites.

"They had hoped to stop our speedy peaceful nuclear activities through software. But, with the grace of God, we discovered the virus exactly at the same spot it wanted to penetrate because of our vigilance and prevented the virus from harming (equipment)," IRNA quoted him as saying.

In Washington, U.S. State Department spokesman Mark Toner told reporters the IAEA report was being studied, "but the key point is that it underscores Iran's continued failure to comply with its international nuclear obligations and also a sustained lack of cooperation with the IAEA."


Associated Press writer Ali Akbar Dareini in Tehran and Matthew Lee in Washington contributed to this report.