The regime has long blamed terrorists for the 11-month-old revolt, and al-Qaeda's endorsement creates new difficulties for the U.S., its Western allies and Arab states trying to figure out a way to help force Assad from power. On Sunday, the 22-nation Arab League called for the U.N. Security Council to create a joint peacekeeping force for Syria, but Damascus rejected it immediately.

In an eight-minute video message released late Saturday, al-Qaeda chief Ayman al-Zawahri called on Muslims to support Syrian rebels.

"Wounded Syria is still bleeding day after day, and the butcher (Bashar Assad) isn't deterred and doesn't stop," said al-Zawahri, who took over al-Qaeda after Osama bin Laden was killed by U.S. special forces last May. "However, the resistance of our people in Syria is escalating and growing despite all the pains, sacrifices and blood."

The United Nations estimates more than 5,400 people have been killed in Syria since the uprising began in March. But that figure is from January, when the U.N. stopped counting because the chaos in the country has made it all but impossible to check the figures.