Iran is continuing to defy UN demands to stop enriching uranium and is expanding its controversial work, the UN nuclear watchdog said in a report.
The IAEA said its ability to monitor Iran's activities had "deteriorated"
The International Atomic Energy Agency report also said Tehran was blocking IAEA efforts to probe suspicious nuclear activities.
The US is now discussing taking further steps against Tehran with the threat of harsher international sanctions.
Iran denied obstructing inspections and said it will "continue to co-operate".
"There are no obstacles to lawful and legal IAEA inspections of Iran's nuclear facilities," said Mohammad Saeedi, the deputy head of Iran's atomic energy organisation.
In March, the UN Security Council imposed a second round of sanctions on Iran for failing to suspend uranium enrichment, prompting Tehran to limit its co-operation.
However IAEA inspectors still regularly visit Iran's atomic facilities under the nuclear Non-Proliferation Treaty.
The White House described the IAEA's latest report as "a laundry list" of Iran's continued defiance of the international community.
The BBC's Jonathan Beale in Washington says the US is determined to step up the pressure, convinced that Tehran is secretly trying to develop nuclear weapons - a charge it denies.
In its report, the IAEA said Iran was operating eight cascades of 164 centrifuges, which are sets of machines used for enrichment at its underground site in Natanz.
It said it was in the process of installing five more. A senior UN official said if Iran continued at the present rate it would have 3,000 centrifuges by the end of June.
At that point, analysts estimate it would take only a further nine to 11 months to produce enough highly enriched uranium for a nuclear bomb.
The report also said Iran had agreed to unannounced inspections and surveillance measures at Natanz but that the IAEA's ability to monitor the nuclear programme had "deteriorated" because of lack of access.
The IAEA document comes amid reports that the US is to complain to IAEA head Mohamed ElBaradei about his proposal that Iran be allowed to keep part of its enrichment programme.
Mr ElBaradei has said Iran possessed "the knowledge about how to enrich" and the focus should now be on preventing industrial-scale production.
There are concerns his comments will weaken UN resolve to punish Iran over its nuclear activities.
Envoys from the US, France, Germany and Britain are expected to visit Mr ElBaradei this week to deliver a formal complaint that the agency chief's comments "were not helpful".