Fighters from an Al-Qaeda-linked group expelled rival rebels from a northern Syrian town after heavy clashes Monday, then quickly moved to eliminate any pockets of resistance by setting up checkpoints on major roads and conducting house-to-house raids in search of opponents, activists said.

The Islamic State of Iraq and the Levant’s seizure of the town of al-Bab is part of a brutal battle between the Al-Qaeda-affiliated group and rebels from Islamist and more moderate factions that has raged across opposition-held territory in northern Syria for the past 11 days. The rebel-on-rebel clashes are the most serious since the Syrian civil war began, and have further muddied an already complicated conflict less than two weeks ahead of a planned international peace conference for Syria in Switzerland.

The pace and scale of the rebel war-within-a-war has eclipsed even that of the opposition’s fight against President Bashar Assad, with at least 700 people killed since the infighting began Jan. 3, according to the Britain-based Syrian Observatory for Human Rights. Many civilians have welcomed the fight against the “Islamic State,” which alienated large chunks of the public with brutal tactics such as kidnappings, extortion and beheadings as it sought to impose its strict interpretation of Islamic law. […]