NATO said the missiles, as well as the radar system, will only be used for defensive purposes 鈥� that is they will only be used to intercept mortars once they have crossed into Turkish territory.

There is a growing sense that the rebels will soon launch a major assault on the Syrian capital. Many extended families are already crammed into small apartments in areas that have been relatively safe so far.

Islamist rebels are among the groups fighting to topple Syrian President Bashar Assad, raising concerns in the West about what their role would be in a Syria without Assad. NPR's Kelly McEvers recently spent some time with these Islamist fighters in the embattled city of Aleppo.

The U.S. is emphasizing the assistance it provides to Syrian refugees, but the Obama administration is still not showing any signs that it's prepared to arm the rebels.