BEIRUT (AP) — Syria's al-Qaida branch on Thursday launched an attack on two predominantly Shiite towns in northern Syria, the militants and a Syrian monitoring group said.

The Nusra Front and several Syrian rebel groups fighting President Bashar Assad's government have besieged and tried to take the towns of Nubul and Zahra since spring 2013.

There are fears that if the towns, strategically important because they lie just off a highway linking the city of Aleppo with the Turkish border, fall into militant hands, there could be reprisals against its Shiite residents. The predominantly Sunni al-Qaida sees minority Shiites as apostates who are worthy of death.

Underscoring the sectarian hatreds, Nusra militants claimed on the group's Twitter feed, to have destroyed Shiite mosques while shelling the towns, which have a combined population of about 40,000 people.

A Syrian monitoring group and a Lebanese Shiite television confirmed the fighting, and said that residents and defense committees were defending the towns.

The fighting is one layer of the complex four-year-old Syrian civil war. It is separate from the battles underway with Islamic State militants.

On Thursday, the U.S. military said they had struck six targets of the Islamic State group in northern and eastern Syria.