The officials, speaking on the condition of anonymity, generally endorsed reports issued by an Iranian resistance group this summer that accused Iran of building facilities for their nuclear programs at the two sites.

U.S. intelligence officials do not believe Iran has made any nuclear weapons. Iran denies having a nuclear weapons program.

In August, officials with the National Council of Resistance of Iran said the sites, once completed, will be a nuclear fuel production plant and research lab at Natanz and a heavy water production plant at Arak. Both sites are in central Iran, south of the capital of Tehran.

The rebel group cited their own sources inside the Iranian government.

The Natanz plant also may include a uranium-enrichment facility, U.S officials said. A heavy water plant at Arak would be part of a plutonium program.

U.S. officials say Iran's lack of fissile material - either enriched uranium or plutonium - remains a key stumbling block for its nuclear goals.

Iran has not declared either site to international monitors, U.S. officials said.

The National Council of Resistance of Iran, based in Paris, is a government-in-exile that advocates violent overthrow of Iran's religious government. Officials say they want to install a democratic government in Iran that protects human rights.

Although the U.S. State Department says the council is a terrorist organization, its members operate freely in the United States, and some in Congress support removing the terrorist label.

Earlier this year, CIA Director George J. Tenet said U.S. intelligence is worried countries like Iran may make "sudden leaps" in their nuclear programs.

"Tehran may be able to indigenously produce enough fissile material for a nuclear weapon by late this decade," Tenet told a congressional committee in March.

Much of the public attention given to Iran's nuclear effort focuses on a power reactor at Bushehr, which is being built with Russian assistance. But the design of the reactor, as well as international agreements for oversight of its operation, are expected to prevent it from being used to make material that can be used in nuclear weapons.

Instead, the primary concern about the reactor is that it will lead to more expertise in nuclear matters in Iran, benefiting its weapons program, U.S. officials say.

Separately, Iran is considering construction of another major nuclear power plant, state-run television reported Thursday.