"Even before Thursday’s bloodshed, however, U.S. and other intelligence officials had justified their reluctance to send materiel support to the rebel movement by citing concerns that militant Islamist groups may be gaining a foothold in Syria. Jordan and other neighbors have resisted efforts by some countries to supply heavy arms, such as antitank and antiaircraft missiles, to rebel forces, two senior intelligence officials said. Regional leaders are beginning to fear that jihadists may gain de facto control over swaths of territory inside Syria, which could be used to launch terrorist strikes elsewhere. They cite indications that extremists affiliated with al-Qaeda-linked groups in Iraq have been making their way across the porous Iraqi border into Syria. Regional intelligence officials who closely monitor movement along the border spoke of “hundreds” of experienced al-Qaeda fighters filtering into the country and warned that the terrorist presence could transform the nature and the goals of the uprising. “They are now inside Syria, and they are well-trained,” said one Middle Eastern official who spoke on the condition of anonymity to discuss sensitive intelligence. “We fear they will turn Syria into what we saw in Iraq after 2004 — a big base for terrorists around the world.”"