Thousands of foreign fighters are flocking to Syria to back the regime in what could be a final offensive in Idlib province, while rebels battle IS in the north.
Activists say regime soldiers have been forced on the defensive on the second day of a major rebel advance to take the city - a claim the regime denies.
Analysis: Pummelled by the international coalition's airstrikes and offensives by Kurdish and Free Syrian Army forces, the Islamic State group is on the defensive in many areas of Syria.
Opposition forces say regime launched surprise attack to cut off rebel-held areas of Aleppo, hours after the UN said it had agreed in principle to a six-week ceasefire.
Fighters say the Islamic State group has been forced from city on Syrian-Turkish border after months of fighting supported by a US-led coalition air campaign.
