The UN nuclear agency said Friday it remained "unable" to say all Iranian activities were peaceful and that Tehran had completed installation at its Fordo site, allowing it to significantly ramp up production.
The IAEA reiterated in a new regular report that it was "unable... to conclude that all nuclear material in Iran is in peaceful activities."
Fordo, which Iran only revealed the existence of to the IAEA in 2009, began operation in 2011, enriching uranium to 20-percent purities -- close to the level needed for the fissile core of a nuclear weapon.
The IAEA said Friday that 2,784 centrifuges were now installed at the facility near the holy city of Qom, but there was no increase in the number operating.
But four new cascades of 174 centrifuges each "having been subjected to vacuum testing, were ready for feeding" with uranium gas, it said.
Once these four new cascades are in operation, monthly production of 20 percent enriched uranium will be around 25 kilos per month, compared to 15 kilos at present, a senior official said on condition of anonymity.
The report also said that because of "activities" spotted at Iran's military base at Parchin means that the IAEA's "ability to conduct effective verification will have been seriously undermined."
Western nations accuse Iran of removing evidence of past nuclear weapons research activities at the base. Alleged evidence of this was outlined in a major IAEA report a year ago.
Iran denies working or ever having worked on a nuclear weapon and says all its atomic activities are peaceful.