DUBAI (Reuters) - Iran denied its hackers attacked American banks, the semi-official Fars news agency reported on Sunday, following a Reuters report that three of the largest American lenders were repeatedly targeted over the past year.

The hacking of websites and corporate networks at Bank of America Corp, JPMorgan Chase & Co and Citigroup Inc began in late 2011 and escalated this year, people familiar with the situation told Reuters.

"We officially announce that we haven't had any attacks," Head of Iran's civil defense agency Gholam Reza Jalali told Fars, when asked about the report.

Reuters reported the hackers targeted the three banks in retaliation for their enforcement of Western economic sanctions against Iran's disputed nuclear program.

Iran says the program is aimed at generating electricity and not, as the United States, Israel and others allege, at making nuclear weapons. A senior Iranian lawmaker accused the head of the United Nations nuclear watchdog on Sunday of passing confidential information about Iran's nuclear activities to Israel.

The Islamic state has beefed up its cyber capabilities after the nuclear program came under attack in 2010 by the Stuxnet computer worm which caused centrifuges to fail at its main enrichment facility.

Tehran accused the United States and Israel of deploying Stuxnet.

Intelligence Minister Heydar Moslehi said in June that Iran had detected a planned "massive cyber attack" against its nuclear facilities, blaming the United States, Israel and Britain.

Jalali said Stuxnet was used against Iran to delay the country's nuclear activities.

"But we see that our activities are successfully continuing ... Our nuclear systems are now immune to such things," Jalali said, according to Fars.

Security experts say Iran's cyber capabilities are not as sophisticated as those of China, Russia, the United States or many of its Western allies.

(Reporting by Zahra Hosseinian)