Iran Iran's chief negotiator at this week's talks aimed at curtailing Tehran's nuclear program has charged the United States with involvement in a cyberattack on the country's nuclear facilities.

Saeed Jalili told NBC News that Iran's investigation has determined that the U.S. was involved in the attack using the Stuxnet computer worm, a virus which targeted centrifuges used to enrich uranium as part of Iran's nuclear program. "I have witnessed some documents that show ... their satisfaction in that [the U.S. was involved]," he said.

The U.N. nuclear watchdog has only limited knowledge of the Stuxnet computer worm, the agency chief said Monday, after reports surfaced claiming the cyberattack had slowed Iran's nuclear programs.

"[The] IAEA's knowledge on this Stuxnet worm is limited," International Atomic Energy Agency chief Yukiya Amano said.

Iran's centrifuges producing enriched uranium have been plagued by breakdowns since a rapid expansion of the process in 2007 and 2008. Israel this month revised its evaluation of Iran's nuclear progress, saying Tehran would not be able to build a bomb for at least four years "because of measures that have been deployed against them". Tehran says the program is aimed at producing nuclear power, not weapons.