By Robin Pomeroy and Ramin Mostafavi
TEHRAN (Reuters) - Iran began loading fuel into the core of its first nuclear power plant on Tuesday, its atomic energy chief said, the last major step to realizing its stated goal of becoming a peaceful user of nuclear energy.
Officials said it showed Iran's nuclear plans were on track despite sanctions aimed at forcing it to curb uranium enrichment which many countries fear is aimed at developing atomic bombs.
But the head of the United Nations' nuclear watchdog called on Iran to address concerns about its true intentions and several European energy companies said they were reducing their dealings with the Islamic Republic due to sanctions.
"This day will be remembered ... because it was the day when fuel was lowered into the core of the reactor," said Ali Akbar Salehi, head of Iran's Atomic Energy Organization.
Amid great media fanfare, fuel rods were transported into the reactor building in August, but they were not inserted into its core and the plant's start-up was delayed due to what were described as minor technical problems.
At a much lower-key news conference, broadcast live from the plant on Iran's coast, Salehi said it would take a further two months to complete the process of lowering 163 fuel assemblies into the core of the reactor and running tests. He said three fuel assemblies had been inserted so far.
The Russian-built 1,000-MW plant will feed Iran's first nuclear power into the national grid early next year, he said.
"If it were in Europe it would supply electricity to about 800,000 or 900,000 people," said Ian Hore-Lacy, of the World Nuclear Association (WNA) industry body.
Iran has denied that the "Stuxnet" computer virus delayed the start-up, although it did infect some computers at Bushehr. Some analysts speculated the worm was designed by Iran's enemies to sabotage the nuclear program.
TALKS STILL STALLED
"Iran's peaceful nuclear activities are going on as scheduled," Foreign Ministry spokesman Ramin Mehmanparast told reporters at his weekly news conference.
Alaeddin Boroujerdi, the head of Iran's parliamentary committee on foreign policy and national security, called the fuelling of Bushehr a victory against sanctions.
"What counts a lot in this process is that America mobilized all its resources across the world to ratchet up the pressure on the Islamic Republic of Iran and they believe that imposing sanctions on us will deter us from making progress," he said.
A U.N. Security Council resolution passed in June, imposing a fourth round of sanctions, renewed a call on Iran to suspend uranium enrichment, something Tehran has explicitly refused to do, saying such activity is its right under international law.
Speaking in Moscow, the U.N. nuclear agency chief urged Iran to allay concerns about its nuclear aims.
"I am requesting Iran take concrete steps, concrete measures toward the full implementation of their obligations," said International Atomic Energy Agency (IAEA) chief Yukiya Amano.
Iran insists it needs to enrich uranium -- material which also can be used to make weapons if refined to a high degree -- to fuel future power stations and a medical research reactor.
Experts say that firing up the $1-billion Bushehr plant will not take Iran any closer to building a nuclear bomb since Russia will supply the enriched uranium for the reactor and take away spent fuel that could be used to make weapons-grade plutonium.
"(Fuelling Bushehr) should not be interpreted as some kind of act of defiance," said Mark Fitzpatrick at the International Institute for Strategic Studies think-tank in London.
"Nobody has asked them to stop on Bushehr. I think it is a big mistake to equate these two issues ... The fact that they have not responded to Catherine Ashton is an important proliferation related issue."
SANCTIONS CLOSE GAS FIELD
EU foreign policy chief Ashton, who represents the "P5+1" powers -- U.N. Security Council permanent members Russia, the United States, China, France and Britain, plus Germany -- has invited Iran for talks in Vienna from November 15 to 17.
Iran has welcomed the offer of talks -- which the powers want to yield a deal curbing Iran's enrichment drive and opening it up to U.N. nuclear inspectors in exchange for a package of benefits -- but has not yet formally replied to the invitation.
"We are following up the issue," Mehmanparast said. "We should reach consensus on the venue and timing as well as the content of the talks."
Similar talks stalled a year ago, leading to the new U.N. sanctions and tighter U.S. and EU measures.
Ashton welcomed the news that a British gas field jointly owned by Iran and energy group BP was set to close due to the sanctions.
The Rhum gas field, 390 km (240 miles) off the northeast of Scotland had been under joint Anglo-Iranian control since 2003 and produced up to 6 million cubic meters of gas a day in the first six months of 2010, or about 1 percent of Britain's peak gas demand forecast for this winter.
Other European oil companies said they were considering whether to keep buying Iranian oil in 2011 due to sanctions.
While U.S. companies have long been prohibited from processing Iranian oil, firms elsewhere face no such ban. But traders say following the sanctions it is harder to pay for Iranian exports in currencies such as the euro and the dollar.