As international inspectors prepare to visit two recently discovered nuclear sites in Iran this weekend, an Iranian opposition group plans to reveal today details of a new site that they say houses equipment for enriching uranium for possible use in nuclear weapons.

Citing sources within Iran, the opposition group also plans to allege that Iranian officials have removed sensitive equipment that was installed at one of the sites, at Natanz, that will be the subject of the visit by the International Atomic Energy Agency.

The opposition group, the National Council of Resistance of Iran, disclosed the existence of the nuclear sites in August and often has revealed reliable information about Iran's efforts to obtain weapons of mass destruction.

Iranian officials have denied the plants are part of a weapons program, arguing they were necessary to wean Iran from its dependence on its vast oil and gas reserves for energy. But the United States has viewed Iran's nuclear program with deep suspicion and has pressed foreign governments -- especially Russia, which is helping to build a reactor at a nuclear plant on the Persian Gulf coast at Bushehr -- to end cooperation with Tehran.

Earlier this month, the Iranian government announced it was planning to develop its own nuclear fuel, a step that could also provide material for weapons. The Russian involvement had previously been considered a safeguard, as Moscow has demanded the return of all spent fuel.

Alireza Jafarzadeh, Washington representative of the opposition group, said the group has information indicating that Chinese and North Korean experts have assisted the Iranian program. About 50 Chinese experts have been observed at a uranium mine at Saghand, and North Korean and Chinese experts supervised the installation of the centrifuge equipment to enrich uranium at a facility near Isfahan, on a road toward a town called Roshan-dasht, he said.

Jafarzadeh said the testing of the centrifuge systems was conducted near Tehran by a front company called Kola Electric Co. at a facility that was officially registered as a watch factory. The facility contains two large halls, each about 4,500 square feet, he said.

Jafarzadeh said the Iranian government in recent months appears to have stepped up its nuclear program in hopes of building a nuclear device between 2004 and 2005. "Their aim is to shift the balance of power in their favor in the region," he said.

Until recently, Iran had rebuffed efforts by the IAEA to examine the two sites. But Mohamed ElBaradei, the head of the U.N.'s nuclear watchdog, will travel to Iran on Saturday and Sunday and urge Iranian officials to sign an "additional protocol" that would permit regular monitoring of Iran's nuclear operations. ElBaradei and a team of experts will visit plants at Natanz, in central Iran, and at Arak, in the southwest.

The Natanz project, started two years ago, is spread over 25 acres, with sections 25 feet underground and protected by eight-foot-thick concrete walls. The Arak facility was started in 1996 and appears designed to produce heavy water necessary for plutonium's use in weapons. The Arak facility, along a river near the central Iranian city, appears to be 87 percent completed and ready for testing in April. The group has information that the government will contend that the Arak plant is designed to produce heavy water for industrial use.

The Natanz site is too large for a country's first enriched uranium facility, experts have said, indicating that Iran may be operating a smaller pilot plant. That could be the new facility near Isfahan disclosed by the group.