The International Atomic Energy Agency, the U.N. unit that monitors nuclear programs, will provide no explanation of the problems. But speculation immediately centered on the Stuxnet worm, a computer virus that some researchers say appears to have been designed specifically to target Iran's centrifuge machines so that they spin out of control.

Iran denies the worm caused any problems.

No country has claimed responsibility for developing the virus, although suspicion has focused primarily on Israel and the United States. James L. Jones, who until recently was President Obama's national security adviser, declined to comment on the worm when asked about it Monday at the Aspen Institute.

The Associated Press first reported on the centrifuge shutdown, which was confirmed by a person familiar with the report. The official said the shutdown is mentioned in a much-anticipated IAEA report expected to be released Tuesday.

U.S. officials did not respond to a request for comment.

Even before the Stuxnet attack, the Natanz facility that houses the centrifuges had not been operating at full capacity, according to experts and U.S. officials.

Olli Heinonen, a former top IAEA official, said Monday at a meeting sponsored by the Arms Control Association that 3,772 centrifuges at the facility were being fed uranium gas and 5,084 machines were idle. "This indicates that there is a problem," he said.

Heinonen also said that Iran appears to have suffered a setback in its efforts to develop a second-generation centrifuge capable of enriching uranium more quickly. Iran's centrifuges are based on a Pakistani copy of a decades-old Dutch design, and Heinonen said Iran may have trouble obtaining the raw materials - such as high-strength carbon - for an upgrade because of international sanctions.