The surge in fighting has raised the prospect of another violent front reopening in the Middle East, at a time when U.S.-backed Israel is fighting Hamas

BEIRUT—Syria’s antigovernment rebels claimed a stunning advance into Aleppo, a major symbolic and strategic prize, in a setback for the government in Damascus and its allies Russia and Iran.Islamist-led insurgents poured into Aleppo two days after launching an unexpected strike from the country’s rebel-held northwest and eight years after the city fell to forces loyal to the government of Bashar al-Assad at the end of a long siege.

The lightning assault on Aleppo by the Islamist militant group Hayat Tahrir al-Sham abruptly redrew the front lines of Syria’s civil war.