Syria's rebels are mainly Sunni, with hard-line brigades emerging as the most powerful fighting groups.
Shiites and other Syrian minority groups have either stayed neutral or sided with President Bashar Assad, fearing for their future should the rebels prevail.
Groups on both sides have targeted civilians.
Also Sunday, Syrian military aircraft bombed Bab al-Hawa, a border crossing to Turkey, said a private Turkish news agency, Dogan, and the Aleppo Media Center and the Local Coordinating Committees, two activist networks.
The news agency said the bombs hit the Syrian side of the crossing, killing or wounding several people, and that several ambulances from the Turkish town of Reyhanli were heading to the border gate to carry the wounded to hospitals.
It was not immediately clear why the area was targeted.