DAMASCUS, SYRIA — The United States encouraged its Syrian Kurdish allies to reach Monday’s landmark deal with the Islamist-led government in Damascus, six sources said, an agreement that could stave off further conflict in northern Syria at a time of uncertainty over the future of U.S. forces deployed there. The deal aims to stitch back together a country fractured by 14 years of war, paving the way for Kurdish-led forces, which hold a quarter of Syria, to merge with Damascus along with regional Kurdish governing bodies. Key details of how this will happen have yet to be spelled out, however.