The International Atomic Energy Agency's latest quarterly report said that Tehran has accelerated the installation of advanced uranium enrichment equipment at its central Natanz plant.
It also outlined further advances at a reactor under construction at Arak which Western countries fear could provide Iran with plutonium if the fuel is reprocessed.
Highly enriched uranium and plutonium can both be used in the explosive core of a nuclear weapon. Iran has denied this is its aim.
Iran has said it intends to install around 3,000 of the new centrifuges at Natanz -- where around 13,500 of the older models are installed -- enabling it to speed up the enrichment of uranium.
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The UN Security Council has passed numerous resolutions calling on Iran to suspend all enrichment and heavy water activities -- the kind being built at Arak --, and has imposed four rounds of sanctions.
Last year additional unilateral US and EU sanctions targeting Iran's oil exports and its financial system began to cause real problems for the Persian Gulf country's economy.
Despite the developments at Natanz, the IAEA report seen noted that Iran has not started operating any new equipment at its Fordo plant.
Fordo is of more concern to the international community, since it is used to enrich uranium to fissile purities of 20 per cent and Natanz mostly to five per cent.
The ability to enrich to 20 per cent is technically speaking considerably closer to 90 per cent, the level needed for a nuclear weapon.
It also "observed that a number of other major components had yet to be installed, including the control room equipment, the refuelling machine and reactor cooling pumps," the IAEA said.