U.S.-trained rebels due to fight Islamic State will fail if they are not also protected from the Syrian army, the main Western-backed political opposition said Thursday, afterWashington said it might lack the legal authority to defend them from Syrian troops.

A fledgling U.S. military-led training program starting in coming weeks aims to train upwards of 5,000 fighters a year to battle the hardline militant group, also known as ISIS, which controls large parts of Syria and Iraq.

But U.S. Defense Secretary Ash Carter said Wednesday that theUnited States did not appear to have clear-cut legal authority to protect Syrian rebels against attacks by Syrian government forces. […]