The International Atomic Energy Agency has said Iran has activated more equipment to enrich uranium more efficiently, violating UN resolutions.
The UN watchdog said a second set, or "cascade", of centrifuges was operating at the Natanz pilot fuel enrichment plant when inspectors visited in July.
The move to enrich uranium to 20% purity means Iran could quickly advance to making weapons-grade material.
The West believes Iran is seeking to build a nuclear bomb. Iran denies this.
The UN Security Council, the US and EU have each imposed sanctions on the Iranian authorities to force them to halt enrichment activities.
Iran has been producing low-enriched uranium (LEU) of about 3.5% purity for some time, and announced in February that it had begun enriching uranium to 20% to make fuel for its Tehran research reactor, which produces medical isotopes. A bomb would require at least 90%.
"The IAEA can confirm that on 17 July, when agency inspectors were at [Natanz], Iran was feeding nuclear material to the two interconnected 164-machine centrifuge cascades," spokeswoman Gill Tudor said.
Ms Tudor said the move was "contrary to UN Security Council resolutions affirming that Iran should suspend all enrichment-related activities".
The centrifuges spin uranium hexafluoride (UF6) gas at high speeds to separate the fissile U-235 atoms from the denser U-238 atoms.
Experts say that using two interconnected cascades increases efficiency by allowing leftover LEU to be re-fed into the machines.
The IAEA said in a report in February that Iran had achieved enrichment levels of up to 19.8%, which added to its concerns about the "possible military dimensions" of its nuclear programme.
Experts say the technical leap required to get to 90% purity from 20% is relatively straightforward, because it becomes easier at higher levels. Going from the natural state of 0.7% purity to 20% takes 90% of the total energy required, they add.
Iran insists its nuclear programme is for entirely peaceful purposes.
Meanwhile, the head of the country's atomic energy organisation announced on Monday that its first nuclear power station at Bushehr would come on stream by September, after years of delays.
"The plant is undergoing the final sets of experiments for detection of any possible failure," Ali Akbar Salehi said. "The preliminary phase will be completed in less than two weeks and the plant will be ready to launch."