Syria's al-Qaida branch says it has launched an attack against two predominantly Shiite towns in northern Syria.

The Nusra Front says on its Twitter account that it began attacking the towns of Nubul and Zahra on Thursday. The towns, with a combined population of about 40,000, have been under siege since the spring of 2013.

A Syrian monitoring group and a Lebanese Shiite television confirmed the fighting, saying the residents are defending the towns, strategically located just off a highway linking the city of Aleppo with the Turkish border.

The predominantly Sunni al-Qaida branch sees minority Shiites Muslims as apostates who are worthy of death.

The fighting is one layer of the complex Syrian civil war and is separate from the battles underway with Islamic State militants.