BEIRUT (Reuters) - Two pro-government Shi’ite Muslim villages in northwest Syria have come under fresh attack from rebel forces which have besieged them for years, prompting retaliatory air strikes, a war monitor said on Sunday.

The villages of al-Foua and Kefraya, which represent the last remaining besieged population in Syria according to the United Nations, were attacked by rebel forces overnight, the Syrian Observatory for Human Rights said.

Fighting on the edge of the villages began early morning Friday, shortly after air strikes carried out by pro-government war planes on the village of Zardana in northeast Idlib killed at least 51 people.

The overnight attack on al-Foua and Kefraya was the fiercest in three years, the head of the Britain-based Observatory Rami Abdelrahman said.

On Sunday heavy air strikes struck a number of opposition-held villages near al-Foua and Kefraya, including the town of Binnish, and further afield in Idlib province, the Observatory said.

State news agency SANA said al-Foua and Kefraya’s defense forces repelled the dawn attack.