US hacked al-Qa'ida sites, says Hillary Clinton

US cyber-experts hacked al-Qa'ida propaganda online in Yemen, changing ads that bragged about killing Americans into ones that showed the death toll of the terror group's attacks against Yemenis, Hillary Clinton said.

Secretary of state Mrs Clinton said the cyber-attack was launched by US State Department specialists who patrol the web and social media to counter al-Qa'ida's attempts to recruit new followers.

It was a rare public admission of the ongoing covert cyber war against extremists.

Speaking in Tampa, Florida, alongside special operations chief Admiral Bill McRaven, Mrs Clinton said the effort was part of a multi-pronged attack on terrorism that went beyond raids like the one that killed Osama bin Laden to include diplomats working alongside special operations forces to shore up local governments and economies and train regional forces.

"Within 48 hours, our team plastered the same sites with altered versions of the ads that showed the toll al-Qa'ida attacks have taken on the Yemeni people," Mrs Clinton said.

"Extremists are publicly venting their frustration and asking supporters not to believe everything they read on the internet."

She said the cyber-attack was launched by an inter-agency 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-Qa'ida's attempts to recruit new followers.

"Together, they will work to pre-empt, discredit and outmanoeuvre extremist propaganda," Mrs Clinton said.

Offensive attacks on extremist sites are generally attributed to the Pentagon's US Cyber Command, though seldom acknowledged publicly.

The target of the attack, Yemen's al-Qa'ida in the Arabian Peninsula, is considered one of al-Qa'ida's most dangerous offshoots.

Yemen was the launching pad for three foiled al-Qa'ida attacks on US targets: the Christmas 2009 attempt to down an American airliner over Detroit with an underwear bomb and the sending of printer cartridges packed with explosives to Chicago synagogues in 2010.

In the past month the CIA thwarted yet another plot by AQAP to destroy a US-bound airliner using a bomb which could have been undetectable by conventional airport scanners.