BEIRUT — Rebels captured part of a massive Syrian army base outside the embattled northern city of Aleppo, tightening the opposition’s grip on areas close to the Turkish border, activists said Monday.

The rebels also killed 13 soldiers in an ambush near a strategic northern town along a road linking Aleppo, the nation’s largest city and business hub, with Damascus, and captured 20 soldiers and police officers at a major post on the highway linking the central town of Salamiyeh with the northern city of Raqqa, activists said.