Washington says it sees no "proliferation risk" from the launch of Iran's first nuclear power plant.
Russia, which built the plant, began loading nuclear fuel into the facility in the Persian Gulf city of Bushehr on August 21.
U.S. State Department spokesman Darby Holladay told news agencies that Moscow's agreement to supply nuclear fuel and remove spent fuel rods minimized the risk they would be used to make nuclear weapons.
"Russia's support for Bushehr underscores that Iran does not need an indigenous enrichment capability if its intentions are purely peaceful," Holladay told the Reuters news agency.
Iran says it's interested only in producing peaceful nuclear energy, but Western countries suspect Tehran of developing a secret nuclear-weapons program.
Work on the plant was begun by the German company Siemens in 1974, but stopped after the Islamic Revolution in 1979. Moscow revived the scheme in the 1990s.
Speaking at the plant's launch, Russian nuclear agency chief Sergei Kiriyenko told Russian television the project was unique for combining German and Russian nuclear technology. "It can be boldly said that from a technological point of view, no one else would have been able to do it. That's why it's a matter of pride for us," Kiriyenko said.
Washington has criticized Russia for building the plant despite international sanctions against Iran. But Moscow says it's seen no proof of a secret arms program.
Russia insists its deal with Iran to take back the spent fuel -- which could be used to make weapons-grade plutonium -- means it can't aid Iranian efforts to build nuclear weapons.
In June, Moscow supported a UN Security Council resolution imposing a fourth round of sanctions against Iran.
Despite Washington's mild reaction to the startup at Bushehr, State Department spokesman Holladay told the AFP news agency that oversight of the plant by the International Atomic Energy Agency (IAEA) "shouldn't be confused with the world's fundamental concerns with Iran's overall nuclear intentions, particularly its pursuit of uranium enrichment."
Iran disclosed the existence of a second enrichment plant last year. In February, Tehran said it was enriching uranium to a level of 20 percent purity, well above the previously announced 3.5 percent level and just under the amount needed for weapons-grade levels.
The head of Iran's nuclear energy agency, Ali Akbar Salehi, said on August 21 that Tehran hadn't wanted to seek to enrich its own fuel, but that the IAEA turned down a request to help supply fuel from abroad.
"We have said over and over that we are not intending to convert all our uranium into 20 percent enriched uranium," Salehi said. "We will go as far as [necessary until] our needs are met."
Iran says it's enriching uranium for nuclear reactors to eventually generate around 20,000 megawatts of electricity.
The launch of the Bushehr plant prompted Israel to urge the international community to prevent Iran from further advancing its nuclear program. The Israeli Foreign Ministry said in a statement, quoted by Israel Radio today, that Tehran's actions are in ongoing violation of Security Council and IAEA resolutions, and that it should be stopped from continuing to enrich uranium.
Iran is Israel's top security concern, and Israeli officials say a nuclear-armed Tehran would threaten the country's own existence.
In another development today, Iranian President Mahmud Ahmadinejad inaugurated the country's first domestically built unmanned bomber drone, calling it an "ambassador of death" to Iran's enemies. The aircraft can carry up to four cruise missiles and will have a range of 1,000 kilometers.