The United States renewed charges Friday that Iran, despite its continued denials, has an active nuclear weapons program. U.S. officials say their contention is supported by new satellite pictures of two suspect sites in central Iran.

The State Department says the photos "reinforce already-grave" U.S. concerns that Iran has a secret nuclear weapons program.

The commercial satellite imagery carried by CNN and other U.S. news outlets Thursday shows two sites in central Iran one, a complex near the town of Arak described as a heavy-water facility for a plutonium production, and the other an apparent uranium enrichment plant outside the town of Natanz.

At a briefing here, State Department spokesman Richard Boucher told reporters the latter facility was being built partially underground, and as such is inconsistent with Iran's claims that its nuclear intentions are peaceful:

"It appears from the imagery that a service road, several small structures, and perhaps three large structures are being build below grade, and some of these are already being covered with earth," said Mr. Boucher. "Iran clearly intended to harden and bury that facility. That facility was probably never intended by Iran to be a declared component of the peaceful program. Instead Iran has been caught constructing a secret underground site where it could produce fissile material."

Mr. Boucher said the United States had been aware of the two sites for some of months and had discussed the activity there with friends and allies, who he said share the Bush administration's concerns.

He said Iran had "repeatedly rebuffed" requests by the International Atomic Energy Agency to visit the sites, though an Iranian spokesman who denied they are connected to a weapons program said Friday IAEA inspectors would be allowed to go there in February.

The United States has long accused Iran of trying to acquire nuclear weapons. And President Bush in his State-of-the-Union address last January cited what he said was Tehran's "aggressive" pursuit of weapons of mass destruction in including Iran, along with Iraq and North Korea, in an "axis of evil."

With help from Russia, Iran is building a nuclear power reactor at Bushehr on the Persian Gulf and is reportedly negotiating with Moscow for additional power plants.

Spokesman Boucher said even though the Bushehr plant would be subject to international safeguards, Iran would derive weapons know-how from it, and he said there is no economic rationale for Iran's nominally-civilian nuclear program.

"These facilities are not justified by the needs of Iran's civilian nuclear program," said Mr. Boucher. "There is no economic gain for a state that's rich in oil and gas like Iran to build costly nuclear fuel-cycle facilities. I'd point out that Iran flares more gas annually than the equivalent energy its desired reactors would produce."

The United States has tried to convince Moscow to end its role in the Bushehr project, which has been underway in fits and starts since before Iran's 1979 Islamic revolution, and is expected to finally come on line sometime next year.