Diplomatic sources confirmed on Monday that Iran had taken the first step in a process that could yield fuel for atomic reactors or bombs.
It also postponed talks with Russia to discuss a proposal to process fuel on Russian soil for Iran's nuclear stations.
Gholamhossein Elham, the Iranian government spokesman, said on Monday that Tehran would restart enrichment activity that were suspended for more than two years under Western pressure.
On Saturday, Mahmoud Ahmadinejad, the Iranian president, rejected US and European insistance to resume a freeze on the country's nuclear programme and hinted that Iran might withdraw from the treaty.
Then on Sunday, Iran's foreign minister played down Ahmadinejad's threat to withdraw from the NPT, saying Iran had no intention of pulling out of the treaty.
On Monday morning, Elham repeated Iran's demand that Western countries should recognise Iran's right to nuclear technology as a signatory to the nuclear Non-Proliferation Treaty.
"We are committed to international treaties to preserve our right," he said. "But if our right was not recognised, there will be no reason to remain committed to international treaties."
Moscow had proposed allowing Iran to enrich uranium in Russia to ease international concerns that it might bomb-grade material at home.
But Elham said the proposal was acceptable only if it was in addition to enrichment facilities in Iran and announced that talks planned for Thursday had been delayed.
Meanwhile Sergei Kislyak, Russia's deputy foreign minister, said Moscow remained prepared to hold talks with Iran over the proposal on Thursday.
Western countries have successfully pushed for Iran to be reported to the United Nations Security Council for failing to convince the world that its atomic programme is exclusively for energy purposes.
Iran says it wants to produce low-grade enriched uranium for use in nuclear power stations.