Russia has announced it will begin loading fuel into Iran's first nuclear power plant next week, a key step in launching the highly sensitive project after years of delays.
The power plant, built by Russia in the Iranian southern city of Bushehr, will be loaded with uranium-packed fuel rods after nearly 40 years of delays, a spokesman for Rosatom, the Russian atomic agency, said on Friday.
"The fuel will be charged in the reactor on August 21. From this moment, Bushehr will be considered a nuclear installation. This will be an irreversible step," Sergei Novikov told the AFP news agency.
Ali Akbar Salehi, the head Iran's atomic energy organisation, said: "We are preparing to transfer the fuel inside the plant next week ... Then we will need seven to eight days to transfer it to the core of the reactor."
A ceremony inaugurating the plant will be held in late September or early October and inspectors from the International Atomic Energy Agency (IAEA), the UN nuclear watchdog, will be invited, Salehi said.
Russia's nuclear agency says it will send its officials to help Iranian specialists lower nuclear fuel rods into the reactor, Al Jazeera's Neave Barker, reporting from the Russian capital, Moscow, said.
"The reactor will not be fully active until mid-September," he said.
Russia had agreed in 1995 to build the Bushehr plant on the site of a project begun in the 1970s by Siemens, a German firm, but delays have haunted the $1bn project amid the standoff over Iran's disputed nuclear enrichment programme.
The United States had urged Russia to delay the process until Iran proves it is not developing nuclear weapons.
However, on Friday, officials in Washington said that the planned launch of the reactor showed that Tehran does not need to pursue its controversial plans for uranium enrichment.
"Russia is providing the fuel, and taking the fuel back out," Robert Gibbs, the White House spokesman, said.
"It quite clearly, I think, underscores that Iran does not need its own enrichment capability if its intentions, as it states, are for a peaceful nuclear programme."
Russia said the Bushehr plant is monitored by the IAEA and has no link with Iran's uranium enrichment programme.
"It is highly unlikely that the Russians did this without the approval of the United States," Al Jazeera's Anand Naidoo, reporting from Washington, said.
"Relations between Moscow and Washington have been improving for some time."
In June, the UN Security Council approved a fourth round of military and financial sanctions against Iran over its nuclear programme, which included arms embargo, financial controls, asset ban on Iranian companies and a travel freeze on individuals.
Western countries continue to accuse Iran of seeking to acquire a nuclear weapon under the guise of its civilian nuclear programme.
Meanwhile, Tehran insisted the drive is entirely peaceful and that it needs nuclear energy for a rapidly expanding population whose fossil fuels will eventually run out.
If started, Bushehr will have an operating capacity of 1,000 megawatts.
Iran, the world's fifth oil producer, has said it wants to build a network of nuclear power plants with a capacity of 20,000 megawatts by 2020 to enable it to export more of its bountiful oil and gas.