According to a FSA commander, the Coalition is coordinating with the SDF to prevent pro-Assad forces from advancing toward Deir-ez-Zor. The thinking is that U.S.-led forces will advance along the Euphrates to Deir-ez-Zor and don’t want interference from Damascus, despite the Assad-Daesh conflict over the besieged city.

Source: https://twitter.com/badly_xeroxed (Cody Roche at Bellingcat)