Iran on Friday rejected U.S. claims that it was developing a clandestine nuclear program and said all its nuclear plants were open to international inspection.

"We have no nuclear activity or study without the knowledge of the International Atomic Energy Agency," government spokesman Abdollah Ramezanzadeh told reporters Friday. "All our nuclear sites are for peaceful purposes and open to IAEA inspection."

Ramezanzadeh's spoke after U.S. officials on Thursday endorsed claims made by an armed Iranian opposition group this summer that two construction sites in central Iran may be used for a clandestine program to develop nuclear weapons.

U.S. intelligence officials do not believe Iran has made any nuclear weapons.

In August, the National Council of Resistance of Iran, the political umbrella group of the Iraq-based armed Mujahedeen Khalq said once completed, the two sites will be a nuclear fuel production plant and research lab at Natanz and a heavy water production plant at Arak, both in central Iran.

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.

"IAEA is aware of all of our nuclear activities. Natanz is not under the ground. IAEA is welcome to inspect the site," Ramezanzadeh said.

The Mujahedeen Khalq receives support from Iraqi President Saddam Hussein and seeks the violent overthrow of Iran's Islamic establishment. It has no popular base inside Iran.

Meanwhile, IAEA officials said Friday the agency was concerned about Iran's plans to study the possibility of building a second major nuclear power plant.

Melissa Fleming, the agency's spokeswoman, said IAEA Director-General Mohamed ElBaradei would visit Tehran at the invitation of the Iranians. That visit originally was planned for this month but was postponed at Iran's request, she said.

A team of IAEA technical experts will accompany ElBaradei, Fleming said. "We have requested to visit these sites and Iran has indicated it would allow such a visit," she said.

Iran's Atomic Energy Council ordered a feasibility study on a second plant as the country's first nuclear power station, built with Russian help at Bushehr in the south prepares to go on line next year, Iranian state-run television reported Thursday.

Both Russia and Iran insist that the plant will be used for civilian purposes only and open to international inspection, but the United States says the Bushehr facility could help advance Iran's alleged weapons program.

It was not clear if Russia will be involved in the construction of the new plant. Russia said in September that it has no other nuclear programs with Iran. The Kremlin has floated preliminary plans to help Iran build five more nuclear reactors over the next 10 years.

However, the Russian news agency Interfax quoted Atomic Energy Minister Alexander Rumyantsev as saying in September that Bushehr is the only actual nuclear program Russia has with Iran.

The Bush administration has offered Russia economic incentives to abandon the Bushehr project, but the Russians have not accepted the offer. Russia has denied consistently it is helping Iran develop nuclear weapons or with its missiles program.

In September, Russia drew up a plan for the return of spent nuclear fuel from Bushehr, seeking to allay U.S. concerns that the fuel could be used by terrorists and others to build weapons of mass destruction.

The Bushehr plant was begun by the West Germans but was interrupted during the 1979 Islamic revolution. It's worth about $800 million to Russia, which has been reluctant to abandon the project both for economic reasons and matters of international prestige.

Meanwhile, Iran's Atomic Energy Council has approved a broad plan to dramatically increase the country's nuclear energy capabilities by 2021, a newspaper reported Thursday.

"The council approved (a plan stipulating) that the share of electricity provided by nuclear energy should reach 6000 megawatts by 2021," the daily Mardom-Salari, or Democracy, reported. It gave no further details. Iranian atomic energy officials were not available for a comment.