Gholamreza Aghazadeh, head of the Atomic Organization of Iran, also described a European offer to shift Iran's contentious nuclear enrichment program to Russia as "flawed," adding that Moscow has not even approached Tehran on the idea.
Iran is standing its ground amid international pressure to cut back on its nuclear activities, particularly uranium enrichment, which can produce material for use in warheads or fuel for nuclear plants to generate electricity.
"For me, there is no doubt that the process of producing nuclear fuel in Iran will be accomplished," Aghazadeh said during a press conference. "There is no doubt that we have to carry out uranium enrichment."
Aghazadeh, who is also an Iranian vice president, gave no date for when the processes would start, but stressed they would do so at some stage.
Despite Iranian denials, the United States claims Iran is trying to build atomic weapons and is pushing for Tehran to be hauled before the U.N. Security Council for possible economic sanctions.
Germany, France and Britain are pressing Iran to give up enrichment and suggested shifting such activities to Russia, where nuclear material would be enriched only to fuel levels and not weapons-grade.
But Aghazadeh said the idea was unacceptable.
"Russia has not presented an official proposal to us. It has just raised ideas which are seriously flawed," he said.
Iran's past experience of joint nuclear cooperation has been poor, said Aghazadeh, who claimed Tehran has 50 tons of UF-6 gas, the feedstock for enrichment, in France's Eurodif uranium enrichment plant, but has been denied use of it.
"Iran can't trust promises by Europeans that it will deliver nuclear fuel," he said. "There is no guarantee that the west will supply us with nuclear fuel."
But in a goodwill gesture, Aghazadeh said "Iran would not inject uranium gas into centrifuges and won't carry out enrichment" during upcoming Iranian talks with European negotiators.
No date has been set for resumed Iran-European talks, which broke off in August, but Aghazadeh said all outstanding issues can still be resolved if the Europeans demonstrate a "political will" to reach an understanding with Iran.
"We don't want to enter open-ended, aimless or floating negotiations," he said.
A senior American diplomat in Vienna criticized Iran's plans.
"It underscores that Iran is today's greatest threat to the proliferation regime" and efforts to keep nuclear weaponry out of the wrong hands, Gregory L. Schulte, the chief U.S. representative to the IAEA, told The Associated Press.
After the Iran-European talks broke down, Iran restarted uranium conversion, a step toward enrichment, at its Uranium Conversion Facility in Isfahan, central Iran.
Iran temporarily froze its enrichment program in November 2004, but the Europeans want it permanently halted. Tehran says it will never give up its right under the Nuclear Nonproliferation Treaty to enrich uranium and produce nuclear fuel.
Iran also plans to build a 360 megawatt light water nuclear power plant based on local technology in Darkhovin, in Khuzestan province in southwestern Iran, Aghazadeh said.
Asked if Iran could build such a plant, he said: "Yes. Those who think Iran can't do so are not aware of Iran's technological capabilities."
Iran is already building a 40 megawatt heavy water nuclear power plant in Arak, central Iran. The Europeans fear Iran may finally use plutonium from the plant to build an atomic bomb and have urged Iran to scrap it. Tehran has rejected the demand saying it won't accept any new restrictions on its nuclear program.
Iran also wants to produce 2,000 megawatts of electricity by building nuclear power plants with foreign help in southern Iran, he added. Iran plans to produce 20,000 megawatts of electricity through nuclear power plants during the next 20 years.
On Friday, IAEA chief Mohamed ElBaradei said the world was losing patience with Iran in the drawn-out negotiations over its nuclear program.
"They are inching forward and I'm asking them to leap forward," the new Nobel Peace Prize recipient told reporters in Oslo. "The international community is losing patience with the nature of that program."
In response, Aghazadeh said: "Iran is also losing its patience with them."