A source inside of Russia claims Syrian militants are firing American-made surface-to-air missiles at Bashar al-Assad's warplanes.

Russia has supplied many of those warplanes, so they see the situation as American weapons shooting down Russian planes.
Russian Gen. Nikolai Makarov said: "We have reliable information that Syrian militants have foreign portable anti-aircraft missile systems, including those made in the USA... it should be cleared up who delivered them."
Prior to Makarov's comments, there had been "unconfirmed reports" of Syrian rebels using shoulder-mounted missiles. This came to light when Syrian rebels claimed to have shot down a fighter jet near the Iraq border in August. Some have suspected the missiles could be U.S. Stingers.
However, there is extant footage of Syrian opposition fighters using old Soviet SA-7 heat-seeking missiles to shoot at aircraft as well.
Thus the bottom line is that confusion reigns, and Russia appears to be using the confusion to try to ascertain exactly where the U.S. stands.