increasingly is some of the more islamist extremist factions fighting and killing more moderate rebels. >> that, it's a problem for the united states. but also the western nations who would like to support the rebels but you have hard core islamists like al qaeda joining the fight against assad in larger and larger number. influence among the rebel is pretty strong. this is nic robertson reporting. >> reporter: the strength of islamist groups simple think theic to al qaeda appears to be growing. a view supported by this former cia officer, who tracked al qaeda in iraq. >> they have found the sweet spot for terrorism and how al qaeda's evolving. >> reporter: the janes defense analysis estimate as approximately 1,000 different rebel groups, close to 100,000 fighters. as many as half with radical leanings, according to other experts. al qaeda's closest allies, isil