Syrian rebels with ties to Al Qaeda make up a growing portion of the Syrian opposition, thereby posing a serious problem for Obama administration officials formulating U.S. Syrian policy.
The Al Qaeda-affiliated group filing ranks in the opposition is called the Nursa Front. They are receiving funds from Al Qaeda supporters throughout the world, and following every military victory they achieve more weapons and fighters come pouring in as well.
Another sticking point for the Obama administration is that the Nursa Front is not just tied to Al Qaeda in a general sense, but is "a direct offshoot of Al Qaeda in Iraq." This means aiding them in any meaningful way could end with individuals in the foreign policy establishment shining a bright and unwelcome light on the conditions that have emerged in Iraq following the U.S. troop-pullout.
Lastly, the presence of Al Qaeda in the Syrian opposition is reminiscent of conditions in Libya, and we've seen the result of Al Qaeda's success there: a consulate destroyed and four Americans dead.
Therefore, even though Bashar al-Assad appears more and more willing to use chemical weapons on his own people and/or foreign factions of the Syrian opposition, the U.S. remains hesitant to involve itself on the side of Al Qaeda.