ISNA quoted an "informed source" as saying "the injection of gas was carried out" in the past week. "We have obtained the product of the second cascade," the source said.
Uranium UF-6 gas is injected into cylindrical centrifuges that spin at supersonic speeds to produce enriched material. The process can make fuel for power plants or material for atomic bombs.
Iran, the world's fourth largest oil exporter, says its nuclear program aims to meet the country’s energy needs but Tehran has failed to convince western powers, who are threatening to impose United Nations sanctions, after Iran failed to halt enrichment work.
US President George W Bush said he was aware of "speculation" that Iran had started enriching uranium in a second network of centrifuges.
"Whether they doubled it or not, the idea of Iran having a nuclear weapon is unacceptable," Mr Bush said.
But Russian Defence Minister Sergei Ivanov said it was too early to speak about Iran being able to produce weapons-grade uranium, and he "did not share the concerns about this".
"These are empty centrifuges, you can't produce anything with them, so to speak about enriching uranium is premature," Mr Ivanov said according to Itar-Tass news agency.
France's foreign ministry spokesman, Jean-Baptiste Mattei, said the report increases the international community's worries about the growth of Iran's capacity to produce fissile material.
While a British Foreign Office spokesman said it was a matter for the International Atomic Energy Agency (IAEA), the UN's nuclear watchdog body, to investigate.
Diplomats said this week Iran had started "dry testing" a network of 164 centrifuges, known as cascades, to go with an original network that yielded Iran's first batch of enriched uranium suitable for power plant fuel.
The first cascade of centrifuges produced a tiny amount of low-enriched uranium in April.
Western intelligence experts estimate Iran remains three to 10 years away from an industrial-scale operation of thousands of centrifuges that could yield enough fuel for nuclear bombs.
Iran faces possible sanctions for failing to halt its enrichment work, as demanded by the UN Security Council.
Iran has shrugged off the threat of sanctions and Iran's chief nuclear negotiator, Ali Larijani, has threatened retaliation, possibly by halting UN inspections of Iranian facilities.
"By imposing sanctions, you yourselves will be hurt more than Iran. Give up these games," an influential Iranian cleric, Ahmad Khatami, told worshippers during Friday prayers.