Iran said on Saturday it has started loading fuel into its Russian-built first nuclear power plant, in the face of stiff opposition from world powers to its controversial atomic programme.

After more than three decades of delay, engineers finally began loading the Russia-supplied fuel into the plant in the southern port of Bushehr, in the presence of International Atomic Energy Agency (IAEA) inspectors, a statement by Iran's Atomic Energy Organisation said.

"Today is a great day and we have entered the physical launch of the Bushehr plant," Fars new agency quoted Sergei Kiriyenko, the chief of Russian atomic agency, Rosatom, as telling reporters when the fuel transfer began.

"It is an important event and we announce that Russia carried out all its commitments as to the commissioning of the Bushehr power plant ... We are pleased with the commissioning of the plant considering the 36-year wait."

Iran's atomic chief Ali Akbar Salehi said it was a "memorable" day for the plant, given its history of "ups and downs."

"We thank Mr Kiriyenko, the Russian government and the (Russian) nation for accompanying the Iranian nation. The Russians have immortalised their names in Iran's history."

Last week, Salehi said the entire transfer of fuel into the reactor would be complete by September 5.

On Friday, Iranian Atomic Energy Organisation spokesman Ali Shirzadian said the one-billion-dollar plant's actual commissioning would come in October or November when the electricity it generates connects to the national grid.

The much-anticipated launch comes despite the fact that Moscow, a long-time nuclear ally, has hardened its position on Tehran's nuclear programme.

In June, Russia backed a fourth round of UN sanctions against Iran over its uranium enrichment, the most controversial part of its atomic programme and which the West believes is aimed at making nuclear weapons, a charge Tehran strongly denies.

Iran says it is enriching uranium to power nuclear reactors so it can eventually generate electricity of around 20,000 megawatts.

Despite being OPEC's second-largest crude oil exporter and having the world's second largest gas reserves, Iran insists it needs nuclear power for when its fossil fuels eventually run out.

On Wednesday, Russian foreign minister Sergei Lavrov said the Bushehr plant would keep Iran "within the regime of non-proliferation."

Russia has supplied 82 tonnes of fuel for Bushehr, but plans to take back the spent material to avoid any misuse.

Mark Fitzpatrick, an expert in non-proliferation at the International Institute for Strategic Studies in London, said Bushehr is not a proliferation risk "as long as it is run to produce power for electricity generation."

"It would be a risk if Iran operated it differently, i.e. for short periods at low-burn up in order to produce weapons-usable plutonium — but in this case the IAEA would know," he said of the UN atomic watchdog.

Work on the Bushehr plant, which is not targeted under UN or other sanctions, began in the 1970s under the rule of the US-backed shah using contractors from German firm Siemens.

The project was shelved when the shah was toppled in the 1979 Islamic revolution, and was revived a decade later under current supreme leader Ali Khamenei.

In 1994, Russia agreed to complete its construction, but since then "technical problems" and squabbling between Moscow and Tehran delayed its completion.

Kiriyenko explained that the Bushehr plant was a "unique project which has no equivalent in the world as several equipment were installed on top of the equipment installed by the Germans."

But fresh doubts over Bushehr were raised after Moscow voted for the latest UN sanctions, followed by Russian President Dmitry Medvedev commenting that Tehran was close to attaining the potential to build a nuclear weapon.

This triggered an angry response from Iran, as Moscow's position has always been that Tehran has the right to peaceful nuclear energy.

Ali Asghar Soltanieh, Iran's envoy to the IAEA, accused Moscow of delaying Bushehr's start-up, saying "it is hard to believe that the technical issues continue to delay the completion of the plant over the past 15 years."

Bushehr has always been seen as a potential target in the event of a military strike by Iran's arch-foes the United States and Israel which have never ruled out military action against Tehran's nuclear programme.