The Assad regime is gaining ground as it takes advantage of infighting between Syrian opposition groups, in a sign of how the rise of extremists could tip the balance in the three-year-old civil war.

Pro-regime forces have consolidated recent gains in and around the northern city of Aleppo, Syria's economic hub. These forces are also geared up to recapture more territory around the city, in a potential major setback for rebels after 18 months of battles to oust the regime from parts of the city that have remained under its...