After decades of delay and sanctions, Iran on Saturday launches its Russian-built first nuclear power plant in the face of Western suspicion that its atomic programme has a covert military agenda.

Iran's nuclear chief Ali Akbar Salehi said last week that "on August 21, the Russia-supplied fuel will be transferred inside the building in which the engine" of the Bushehr power station in southern Iran is located.

Salehi said the fuel transfer would be completed by September 5 and two weeks later the reactor would reach 50 percent of its power-generating capacity, allowing it to be linked to the national grid.

"It will take six to seven months to reach the maximum power level of the plant," he added.

The much-anticipated launch of the more than one-billion-dollar Bushehr plant comes despite Moscow hardening its position on Iran's nuclear programme by backing a fourth round of UN sanctions against Tehran on June 9.

Iran is under sanctions because of its uranium enrichment programme 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 which will eventually generate electricity of around 20,000 megawatts.

On Wednesday, Russian Foreign Minister Sergei Lavrov defended the plant, saying it would keep Iran firmly fixed to peaceful use of nuclear power.

"It is a most important anchor which keeps Iran within the regime of non-proliferation," he told reporters.

"It is fully protected from any proliferation risks whatsoever. This idea is shared by all the leaders of Western countries."

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

But the project was shelved when the shah was toppled in the 1979 Islamic revolution, only to be revived a decade later under current supreme leader Ali Khamenei.

In 1994, Russia agreed to complete the construction and also fuel the plant after Siemens backed out.

But since then "technical problems" and squabbles between Moscow and Tehran have delayed completion of the facility, with the project further suffering from the controversy surrounding Iran's nuclear programme under hardline President Mahmoud Ahmadinejad.

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

It was the first time a Russian leader had spoken so bluntly of the dangers of Tehran's nuclear drive, especially since the Russian position has been that Iran has the right to peaceful nuclear energy.

After the latest sanctions, Iran's envoy to the UN atomic watchdog, Ali Asghar Soltanieh, accused Moscow of delaying Bushehr's commissioning.

"After Russia voted for the new sanctions against Iran at the UN Security Council, we reached the conclusion that the Kremlin's delay in launching the Bushehr power plant is politically motivated," he said.

"Of course there may exist some technical problems... but it is hard to believe that the technical issues continue to delay the completion of the plant over the past 15 years."

The Bushehr plant was always 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.

On Tuesday, Salehi warned that "attacking an international plant is an international crime as the consequences will not be limited to the hosting country but will have a global aftermath."

He spoke a day after former US envoy to the United Nations John Bolton said Israel had lost the opportunity to attack the plant.

A sceptical Bolton, when asked by Fox Business Network about the possibility of an Israeli attack ahead of the plant's opening, said: "I don't think so, I'm afraid that they've lost this opportunity."

Bolton said Bushehr's launch meant Iran had achieved "something that no other opponent of Israel, no other enemy of the United States in the Middle East really has and that is a functioning nuclear reactor."