Russia will load fuel into Iran's first nuclear power plant next week despite U.S. efforts to prevent the country from obtaining nuclear energy until it proves that it's not pursuing a weapons capacity, officials said Friday.

Uranium fuel shipped by Russia will be loaded into the Bushehr reactor on Aug. 21, beginning a startup process that will last about a month and end with the reactor sending electricity to Iranian cities, Russian and Iranian officials said.

A worker tends to equipment in the fuel manufacturing plant at the Isfahan Uranium Conversion Facility, about 440 kilometres south of Tehran. (Caren Firouz/Reuters)

"From that moment the Bushehr plant will be officially considered a nuclear-energy installation," said Sergei Novikov, a spokesman for the Russian nuclear agency, told The Associated Press.

Russia signed a $1-billion contract to build the Bushehr plant in 1995 but it has dragged its feet on completing the project.

Moscow has cited technical reasons for the delays, but analysts say Moscow has used the project to press Iran to ease its defiance over its nuclear program.

Russian officials say, however, that UN sanctions against Iran, including a new, more stringent set approved in June, don't directly prevent Moscow from going ahead with the Bushehr project. It has argued that the Bushehr project is essential for persuading Iran to co-operate with the UN nuclear watchdog and fulfil its obligations under international nuclear non-proliferation agreements.

Russian officials did not say why they had decided to move ahead with loading fuel into the Bushehr plant now.

The uranium fuel used by the Bushehr plant is enriched to a level too low to be used in a nuclear weapon. Iran is already producing uranium enriched to that level — about 3.5 per cent — and has started a pilot program of enriching uranium to 20 per cent. Iran claims it needs the 20 per cent enriched uranium to produce fuel for a medical research reactor, but the move has further heightened international concerns about its nuclear program.

Uranium must be enriched to over 90 per cent to be used in a nuclear warhead.

Iran's semi-official ISNA news agency quoted vice-president Ali Akbar Salehi, who is also the head of the Atomic Energy Organization of Iran, as saying that the country had invited International Atomic Energy Agency experts to watch the transfer of fuel, which was shipped about two years ago, into the Bushehr reactor.

"Fuel complexes are sealed [and being monitored by IAEA]. Naturally, IAEA inspectors will be there to watch the unsealing," ISNA quoted Salehi as saying.

Russia has said that the Bushehr project has been closely supervised by the UN nuclear watchdog, which declined comment Friday. It also says Iran has signed a pledge to ship all the spent uranium fuel from Bushehr back to Russia for reprocessing, excluding a possibility that any of it could used to make nuclear weapons.

Russia has walked a fine line on Iran for years. It is one of the six powers leading international efforts to ensure Iran does not develop an atomic bomb. It has backed UN sanctions, but strongly criticized the U.S. and the European Union for following up with separate, even stronger sanctions.