Iran's nuclear chief has warned that the International Atomic Energy Agency (IAEA) may have been infiltrated by "terrorists and saboteurs".
Fereydun Abbasi-Davani said explosions had cut power lines to a uranium enrichment facility last month shortly before a visit by IAEA inspectors.
He also accused the UN's nuclear watchdog of mismanagement and said it was influenced by "certain states".
On Thursday, the IAEA rebuked Iran for refusing to suspend uranium enrichment.
The 35-member governing board expressed "serious concern" that Iran continued to defy UN Security Council resolutions demanding a halt and had failed to resolve questions about possible weapons development.
Israel's prime minister warned on Sunday that Iran was only six or seven months from having "90%" of what it needed to make a nuclear bomb.
Benjamin Netanyahu told US media that the only way to stop this was for Washington to draw a "red line" on Iran's nuclear activity and declare that crossing it would lead to military intervention.
Iran has insisted that its nuclear programme is solely for peaceful purposes, and warned that it will retaliate if it comes under attack.
In a speech to the IAEA's general conference in Vienna on Monday, the head of the Atomic Energy Organisation of Iran stressed that it had "always opposed and will always denounce the manufacture and use of weapons of mass destruction".
Then in an unusually outspoken attack on the IAEA, Mr Abbasi-Davani said "terrorists and saboteurs might have intruded the agency and might be making decisions covertly"
He cited an incident on 17 August, when power lines from the city of Qom to the underground uranium enrichment facility at Fordo were cut.
The following day, he said, the IAEA sought an unannounced inspection.
"Does this visit have any connection to that detonation? Who other than the IAEA inspectors can have access to the complex in such a short time?"
"It should be recalled that power cut-off is one of the ways to break down centrifuge machines," he added, referring to the equipment used to increase the proportion of fissile uranium-235 atoms within uranium.
But Mr Abbasi-Davani said sabotage attempts against Iran's nuclear programme had failed and would continue to fail.
For his part, the head of the IAEA, Yukiya Amano said the agency was committed to intensifying dialogue with Iran, despite the lack of progress so far. The last meeting on 24 August ended without agreement.
This comes a day before the EU's foreign policy chief, Catherine Ashton, is due to meet Iran's chief nuclear negotiator, Said Jalili, in Istanbul.
Baroness Ashton is the lead negotiator for the so-called P5+1 - the US, UK, France, China, Russia and Germany.