The United States was relegated to the sidelines of the Syrian war on Friday as an all-out assault by Syrian and Russian forces on opposition-held Aleppo threatened to unleash a new wave of refugees and drive U.S.-backed rebels into the ranks of al-Qaida's Syrian branch, U.S. officials and experts said.

Moscow's direct participation in the offensive left fading hope for U.S.-Russian peace efforts, raising the likelihood that U.S. President Barack Obama's successor will inherit a worsening conflict.

Entering its sixth year, the civil war has left 250,000 people dead, uprooted more than 11 million and provided a base for the Islamic State group to launch and inspire attacks in the West.