There’s a good chance those are the same missiles the White House supplied to “vetted” FSA groups.
Via LWJ:
The Islamic State has released photographs showing its fighters using a US-made BGM-71 TOW anti-tank missile against elements of the US-backed Free Syrian Army in the Damascus countryside.
The images were published by the Islamic State’s Wilayat Damascus (State of Damascus) and disseminated on Twitter by its supporters.
In the photo set, many Islamic State fighters can be seen amassing in more than 15 technicals, or armed pickup trucks. The pictures then show the use of the TOW missile on Free Syrian Army (FSA) units. The last few photos show FSA members the Islamic State has taken captive.
While the Islamic State is publicizing its use of the American made weapon, it is unclear if it is the same model that is being supplied to various FSA groups throughout Syria. At least one TOW missile was also recently used by the Al Nusrah Front, al Qaeda’s official branch in Syria, in that group’s recent offensive on Wadi al Daif in the northwestern province of Idlib. [For more on this, see LWJ report Al Nusrah Front uses American-made anti-tank missile in Idlib]
Additionally, many FSA groups have utilized supplies of TOW missiles to assist the Al Nusrah Front or allied jihadist groups in the past. In early Oct, many FSA groups assisted Al Nusrah and Ahrar al Sham in southern Syria with TOWs. Then a few days later, the Hazm Movement posted a video of its fighters utilizing TOWs in support of Jaish al Muhajireen wal Ansar in Aleppo. The Chechen group considers itself to be the Syrian branch of the Caucasus Emirate, an al Qaeda-linked jihadist organization that operates in the North Caucasus.