Iran has begun loading fuel into the core of its first nuclear power plant, state television has reported.
It marks a key stage in the firing-up of the Bushehr plant, which is set to produce electricity from 2011.
Russia will operate the facility in southern Iran, supplying its nuclear fuel and taking away the nuclear waste.
Iran's separate uranium enrichment programme has alarmed Western nations, who distrust Iran's claims it is solely for peaceful purposes.
Iran has been subject to four rounds of UN sanctions because of its nuclear programme.
Experts say that as long as the Bushehr plant is Russian-operated and supervised by the UN nuclear watchdog the International Atomic Energy Agency (IAEA), there is little chance of proliferation.
German engineers began work on the Bushehr nuclear facility in the 1970s. The project has been plagued by delays, but was completed with help from Russia.
Iranian and Russian engineers initially began to move nuclear fuel into the reactor in August, amid great media fanfare, but the work was again delayed - Iran says due to a leak in the central pool of the reactor building.
"Fuel injection into the core of the reactor has begun," state TV announced on Tuesday.
Mohammad Ahmadian of Iran's Atomic Energy Organisation said some 160 fuel rods would be placed in the core of the reactor - a process Iranian media report will take about 20 days.
"The reactor will [then] hopefully be ready to become operational," Mr Ahmadian said - though it will then take a month or two to begin producing electricity at full capacity.
Foreign Ministry spokesman Ramin Mehmanparast said Iran's nuclear programme was "progressing well according to the fixed timetable despite all political pressures and sanctions", reported official Iranian news agency Irna.
The uranium fuel Bushehr will use is well below the enrichment level needed for a nuclear weapon.
Weapons-grade uranium must be enriched by more than 90%. In contrast, the uranium at Bushehr is enriched by 3.5%.
In fact, there are strong suspicions that the West has given a green light to the 1,000-megawatt reactor being fired up in a behind-the-scenes deal with Russia, says the BBC's Tehran correspondent Jon Leyne.
Under this deal, it is believed that Washington agreed to the opening of the reactor in return for Russian support for new sanctions on Iran.
The real danger perceived by the West and Israel is elsewhere, our correspondent says - the production by Iran of its own uranium enriched to fuel grade.
It has begun a pilot programme to enrich uranium to 20% - which it says is needed for a medical research reactor.
Some governments fear Tehran wants to build a nuclear weapon, but Iran insists its plans are for peaceful energy production.
The "P5+1" powers - UN Security Council permanent members Russia, the US, China, France and Britain, plus Germany - have invited Iran for talks in Vienna from 15-17 November.
Iran has indicated it would be willing to attend negotiations, but says it needs to have a say in what will be on the agenda. It has not yet given a formal response.