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.
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.