BEIRUT (AP) — A Syrian rights group says that rebels are pounding two besieged Shiite towns in the northern province of Aleppo.

The British-based Syrian Observatory for Human Rights said Sunday that fighting was flaring up on the outskirts of the towns of Nubul and Zahra. Fighters seeking the overthrow of President Bashar Assad have besieged the towns for months and tried to force out his troops there.

A hardline Sunni brigade warned last week they would punish Shiites for harboring the forces, suggesting the towns' population of some 40,000 is at risk.

The fighting underscores the growing sectarian nature of the two-year old uprising against Assad's regime. Lebanese and Iraqi Shiites have joined pro-Assad forces. Foreign Sunni fighters have joined bands of predominantly Sunni Syrian rebels.