US Secretary of State Hilary Clinton has revealed that the US State Department has taken to hacking al-Qaeda\'s websites in Yemen and filling them with US propaganda. In a rare public admission of the covert cyber war against extremists, Clinton said cyber experts based at her department hacked Yemeni tribal websites, and took down messages about killing Americans. \"Within 48 hours, our team plastered the same sites with altered versions of the ads that showed the toll al-Qaeda attacks have taken on the Yemeni people,\" she said on Wednesday. In response \"extremists are publicly venting their frustration and asking supporters not to believe everything they read on the Internet,\" she said. Clinton described the cyber effort as part of a larger, multi-pronged attack on terrorism that goes beyond attacks like the Navy SEAL raid that killed Osama bin Laden to include the propaganda battle, and the longer, slower campaign of diplomats. Clinton, one of president Barack Obama\'s most high-ranking officials, was speaking alongside admiral Bill McRaven, head of the US Special Operations Command, at a conference of hundreds of US and international special operations commanders. Their joint appearance was seen as a tacit message to their sometimes warring tribes of troops and diplomats that they must cooperate. Clinton said the cyber attack was launched by an interagency group of specialists, including diplomats, special operators and intelligence analysts, housed at the State Department. Called the Centre for Strategic Counterterrorism Communications, its experts patrol the Internet and social media to counter al-Qaeda\'s attempts to recruit new followers. \"Together, they will work to pre-empt, discredit and outmaneuver extremist propaganda,\" Clinton said. Offensive attacks on extremist sites are generally attributed to the Pentagon\'s Cyber Command, though seldom acknowledged publicly.