An anonymous official with the U.S. Central Command told Voice of America News that the U.S. military has been giving Russia advance notice of airstrikes in Syria ever since a strike from the U.S.-led coalition struck Syrian government forces in September.

The source said these warnings were issued using a special safety hotline, which was originally established to avoid midair collisions between Russian and coalition aircraft over Syria.

Oddly enough, the hotline was used to give the Russians advance notice about the strike near Deir Ezzor that killed Syrian troops in September, due to the large number of planes involved, and frequent Russian use of the Deir Ezzor airfield.

The official who spoke to VOA News said the Russians have also used the hotline to tip off the U.S. military about airstrikes in Syria, but much less frequently, and the locations given by the Russians are more vague.

The Russians issued a stern public warning against U.S. bombing of Syrian forces after the attack, and went so far as to imply they would shoot down American planes that threatened Syrian forces.

They even threatened the U.S. on Twitter: “USA wonders why Russia would want to carry the S-300 to Syria? Because you never really know what kind of assistance terrorists might get.” The Tweet was illustrated with a goofy photo of White House spokesman Josh Earnest, and portrayed as a joke… except it also wasn’t a joke, because Russia declared it would “take every defensive measure necessary to protect its personnel stationed in Syria from terrorist threats.”