On April 5, video surfaced of Harakat Hazm, a relatively new rebel group (its formation was announced on January 26), utilizing TOW missiles in combat with Syrian regime forces.

https://www.youtube.com/watch?v=0L2uGyD7M04

The TOW is supplied through various military assistance agreements to multiple countries in the region, including Saudi Arabia and Turkey.  While the US has publicly demurred from supplying the rebels with heavy weapons, the Saudis have displayed no such compunction.  According to Oryx Blog, TOWs can be found on the Lebanese black market, just over the border from Syria.  Iran also produces the Toophan (Typhoon) ATGM, which is a reverse-engineered TOW, but so far there has been no evidence of pro-Assad forces using either TOWs or Toophans in Syria, making it unlikely that these were captured from regime or Hezbollah forces.

The use of advanced, US-designed ATGMs (albeit Cold War technology), is yet another escalation in the current conflict in Syria, that has shown no signs of slowing down.

It also has spilled over the border into Iraq, including the proliferation of these heavy weapons.  Saudi Arabia started channeling Croatian weapons to the FSA and allied groups in 2013, through Jordan.  Among these weapons were RBG-6 rotary grenade launchers (similar to the US’ Mk 32) and M-79 “Osa” 90mm anti-tank rocket launchers.  By March, 2013, these weapons were already being seen in the hands of non-FSA Islamist rebel groups, including Ahrar al-Sham.

On March 8, 2014, pictures surfaced of what appears to be M-79s being used by ISIS against Iraqi Security Forces.

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This image has also been posted, depicting an ISIS fighter with what appears to be an RBG-6.