Two al-Qaeda-linked groups are known to operate in Syria - Jabhat al-Nusra, also known as Nusra Front, and the Islamic State of Iraq and the Levant. The groups, which have both been designated terrorist organisations by the United States, have been bolstered by the influx of thousands of foreign fighters from across the Muslim world as well as Europe and North America who have flocked to Syria to take up arms against Assad.