Some contributors to the messages seen on the password-protected, invitation-only militant chatrooms this week called on al Qaeda's North Africa arm to "deter" France by staging armed attacks, the SITE service said.
The anonymous threats, presented by the sites as comments by individuals, carry none of the weight of published audio or video statements by Osama bin Laden's al Qaeda or similar established armed groups seeking to attack Western targets.
But Western security officials are concerned that sermons, discussions and videos published on online forums and social networking sites are increasingly used by militants to encourage sympathisers to attempt attacks.
Individuals radicalised online or via email have carried out a handful of attacks in United States and Britain since 2009.
"Sarkozy, leave our sisters alone or we will truly be obligated to act on our words," wrote one participant with the online identity of "Jabeen of Abdullah Azzam", writing on the Arabic-language Shumukh al-Islam forum, SITE reported.
"Declare a war against crusader France!" wrote "al-katebat al-khdra" (the green battalion) on Shumukh al-Islam. "France with such action brought destruction and woes on itself."
France's ban on full face veils, a first in Europe, went into force on Monday, making anyone wearing the Muslim niqab or burqa in public liable to a fine of 150 euros or lessons in French citizenship.
The law has provoked a limited backlash in France where a strict separation of church and state is seen as central to maintaining a peaceful civil society. But it has been widely criticised by Muslims abroad as harming their religious freedom.
Unlike most of the ban's Muslim critics, the online militants favour a violent response to the law.
Participant "Abuhamza" called on al Qaeda in the Islamic Maghreb to attack France, which he said was positioning itself as a larger enemy of Islam than the United States.
Shumukh forum members posting in an April 12, 2011 discussion called on bin Laden to carry out attacks in France and proposed the creation of a fund to plan and execute attacks against France and French interests, SITE said.
Unlike Britain and Spain, France has never been attacked by al Qaeda at home, despite being a NATO member that took part in the invasion of Afghanistan and still having troops there.
In a statement in October 2010, bin Laden condemned the ban on full-face Islamic veils passed by French constitutional authorities earlier that month and said France deserved to be attacked for "injustices" against Muslims.
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