Syrian activists say rebels and al-Qaida militants are attacking two predominantly Shiite Muslim villages in northern Syria that have been besieged by hardline opposition fighters for some 18 months.

The Local Coordination Committees, which is an activist collective, and the Britain-based Syrian Observatory for Human Rights monitoring group say the attack on the pro-government villages of Nubul and Zahra began overnight.

The Observatory says Islamic rebel groups and fighters from the al-Qaida-affiliated Nusra Front seized an area south of Zahra on Sunday. It says at least eight rebels and several pro-government fighters have been killed in the clashes.