Iranian and Russian engineers began loading fuel into Iran's first nuclear power plant yesterday. The event was a major milestone as Tehran forges ahead with its atomic programme despite UN sanctions.

The week-long operation to load fuel into the reactor at the Bushehr plant in southern Iran ends years of foot-dragging by Russia, which signed a £640m contract in 1995 to build the plant but delayed its completion several times.

Russia is one of six world powers leading efforts to ensure Iran does not develop a nuclear weapon and Moscow says the Bushehr project is essential for persuading Iran to co-operate. But the US disagrees, saying Iran should not be rewarded while it defies UN demands. Iran insists its programme is only for energy production.

"The start-up operations will be a big success for Iran," conservative MP Javad Karimi said in Tehran. "It also shows Iran's resolve and capability in pursuing its nuclear activities."

Russia has pledged to safeguard the site and prevent spent nuclear fuel from being shifted to a possible weapons programme.

The first truckload of fuel was taken from a storage site to a fuel 'pool' inside the reactor building. Over the next 10 days, 163 fuel assemblies – equal to 80 tons of uranium fuel – will be moved inside the building and then into the reactor core. It will then be at least another month before the 1,000-megawatt light-water reactor is pumping electricity to Iranian cities.

The Bushehr plant is not considered a proliferation risk because the deal commits Iran to allowing the Russians to retrieve all used reactor fuel for reprocessing. Spent fuel contains plutonium, which can be used in weapons.

Of greater concern to the west are Iran's stated plans to build 10 new uranium enrichment sites inside mountain strongholds. Iran said recently it would begin work on the first one in March, in defiance of UN sanctions.