The deputy head of Iran's atomic energy agency, Mohammad Saidi, said the UN nuclear watchdog has already been informed of the step, which risks creating further strains in talks with European negotiators.
"In a letter, the IAEA (International Atomic Energy Agency) has been informed that Iran will start research on the technology of nuclear fuel in a few days, with the cooperation and coordination of the agency," Mr Saidi told state television.
"We think our experts have undergone lots of losses during this period (of suspension). Many of our researchers have lost their jobs," he added.
Mr Saidi did not specify exactly what the research concerned, but said that the Islamic republic had "voluntarily" suspended such activities for about "the past two-and-a-half years."
Ahmadinejad confirms
Just hours later, Iran’s hardline President Mahmoud Ahmadinejad, said that Iran would not "step back" on its decision to resume nuclear fuel work, according to state television.
"Our country will go forward on the nuclear path with patience, wisdom and planning," Mr Ahmadinejad said after a parliament session on the state budget.
"In the nuclear matter we have the most coherent diplomacy and the whole regime is acting with full coordination, he said.
"We will not make a step back on our path."
Iran's student-run news agency ISNA further quoted Mr Ahmadinejad as rejecting Western influence on Iranian policies.
"They (Western countries) are so rude that if we allow them they will tell us to shut down all our universities, whereas research has no restrictions or red lines," he said.
"We cannot base our national interest on their policy."
IAEA seeks clarification
Iran announced in October 2003 that it was temporarily suspending uranium enrichment, a process that can be used to create nuclear fuel for reactors and also the cores of atomic bombs.
Diplomats have said that were Iran to resume enriching uranium it would deal a fatal blow to the negotiating process, already fragile after Tehran restarted uranium conversion last year, the precursor step to enrichment.
In a statement confirming receipt of the letter, the IAEA said its director general Mohammed El-Baradei "recalls the importance placed by the IAEA Board that Iran maintains its suspension of all enrichment-related activity as a key confidence-building measure."
It said "he continues to call on Iran to take the steps the IAEA requires to resolve outstanding issues regarding the nature of Iran's nuclear program".
However, Mr Saidi insisted that the decision was not linked to the production of nuclear fuel.
The IAEA said it is seeking clarifications from Iran as to the "implications" of the decision.
France has called on Iran to reverse its move, saying if Iran was to observe a suspension on enrichment it also had to halt research.
"We would like Iran to abide by the suspension of all activities related to the enrichment and reprocessing... which includes centrifuges and research," foreign ministry spokesman Jean-Baptiste Mattei said.
US threatens action
The US has threatened to seek international action against Iran if it resumes nuclear fuel research.
US State Department spokesman Sean McCormack accused the Iranians of doing a "bob and weave" in negotiations to persuade them to halt uranium enrichment activities that could lead to a nuclear bomb.
"Our view is that if Iran takes any further enrichment-related steps, the international community will have to consider additional measures to constrain Iran's nuclear ambitions," Mr McCormack said.
But Mr McCormack said Tehran's general evasiveness on the issue made it necessary to ban research as well.
The US has been hoping to refer Iran to the UN Security Council for possible sanctions.
Mr McCormack would not say what new measures the Americans might seek but suggested the showdown was coming to a head.
Russian delegation
Meanwhile, a delegation from Moscow is to visit Tehran on Saturday amid continued Russian efforts to break the deadlock between Iran's insistence on maintaining its right to enrichment and EU demands it renounces the practice.
Iran is set to have new talks with EU negotiators on January 18 but both sides have acknowledged that wide differences remain.