Hundreds of foreign fighters have abandoned rebel ranks in northern Syria as frustration rises over bloody infighting there – a trend that suggests declining enthusiasm among hardline Sunni Muslim militants participating in Syria’s torturous civil war and raises concerns among western security officials that these combatants may head to other countries.

The outflow of foreign militants is still small, rebels and activists say, but illustrates the disillusionment many appear to feel as they spend more time fighting each other than the regime of President Bashar al-Assad.