BEIRUT — Backed by Turkish tanks and reports of air strikes, Turkey-allied Syrian rebels clashed with Kurdish-led forces in northeastern Syria in a new escalation that further complicates the already protracted Syrian conflict. If confirmed, it would be the first Turkish air strikes against Kurdish allied forces on Syrian soil. The new escalation highlights concerns that Turkey’s incursion into Syria this week could lead to an all-out confrontation between Ankara and Syrian Kurds, both American allies, and hinder the war against the Islamic State group by diverting resources. Turkey has long suspected the Syrian Kurdish People’s Protection Units, or YPG, of being linked to Kurdish insurgents in its own southeast, which it labels as a terror group.