BEIRUT — Syrian Kurds are preparing a plan to declare a federal region in the area they control across northern Syria, saying Wednesday it is a model for a more decentralized government in which all ethnic groups would be represented. Although the idea might seem like a way forward after five years of civil war, it faces big obstacles: It was promptly dismissed by the government of Syrian President Bashar Assad and the rebels who oppose him, both fearing it would lead to a partition of the country. Turkey also opposes it, wary of the growing Kurdish influence in the border region of northern Syria and its effect on its own Kurdish minority. But Ahmad Araj, a Kurdish official in northern Syria, insisted that a federal system containing such a region, which would effectively combine three Kurdish-led autonomous areas, is in fact meant to preserve national unity and prevent Syria from breaking up along sectarian lines. “After all the blood that has been spilled, Syrians will not accept anything less than decentralization,” Araj said. By making the announcement as U.N.-sponsored peace negotiations take place in Geneva, Syria’s main Kurdish faction was trying to become a major player in whatever central government emerges from the war.