Cyber attackers have targeted Iranian infrastructure and communications companies, disrupting the Internet across the country, a state official was quoted as saying on Wednesday. Iran, the world's No. 5 oil exporter, has tightened cyber security since its uranium enrichment centrifuges were hit in 2010 by the Stuxnet computer worm, which Tehran believes was planted by arch-adversaries Israel or the United States. "Yesterday we had a heavy attack against the country's infrastructure and communications companies which has forced us to limit the Internet," Mehdi Akhavan Behabadi, secretary of the High Council of Cyberspace, told the Iranian Labour News Agency. "Presently we have constant cyber attacks in the country. Yesterday an attack with a traffic of several gigabytes hit the Internet infrastructure, which caused an unwanted slowness in the country's Internet," he said. "All of these attacks have been organised. And they have in mind the country's nuclear, oil, and information networks." Israeli officials have threatened military action against the Islamic Republic's nuclear energy sites if Western sanctions on Tehran's banking and oil sectors do not persuade it to shelve its disputed atomic programme. Western powers suspect Iran is trying to develop the means to produce nuclear weapons. Tehran says it is enriching uranium only for civilian energy. Last month a commander in Iran's elite Revolutionary Guards said it was prepared to defend itself in case of a "cyber war" and deemed it more dangerous than a physical confrontation. Iranian authorities said in April that a computer virus was detected inside the control systems of Kharg Island - which handles the vast majority of Iran's crude oil exports - but the terminal remained operational. Iran maintains one of the world's largest Internet filters, blocking access to tens of thousands of websites on the grounds that they are criminal or immoral. Sites expressing anti-government views are routinely barred. Many of the Internet restrictions date back to the use of sites such as Facebook and YouTube to rally and publicize mass anti-government protests that erupted after the disputed 2009 re-election of President Mahmoud Ahmadinejad. Iranian riot police clashed on Wednesday with demonstrators and foreign exchange dealers in Tehran over the collapse of the country's currency, which has lost a third of its value against the dollar in a week, witnesses said. Police fired tear gas to disperse demonstrators who shouted slogans against Ahmadinejad, saying his economic policies had caused the currency crisis.