Updated

Rebels and Al Qaeda militants took over at least three government checkpoints Sunday around a pair of military bases in northwestern Syria, activists said.

The positions that fell to opposition fighters in heavy fighting are located around the Wadi Deif and Hamidiyeh bases outside the town of Maaret al-Numan in Syria's Idlib province. Rebels have been trying to capture both facilities since 2012, but have not been able to take either despite several attempts.

The Britain-based Syrian Observatory for Human Rights and the Local Coordination Committees activist collective both reported the opposition advances Sunday.

Observatory director Rami Abdurrahman said the rebels and fighters from the Al Qaeda-affiliated Nusra Front seized three checkpoints near the bases. He said at least 15 pro-government troops and eight opposition fighters have been killed in the clashes.

The Local Coordination Committees said the rebels took over a fourth checkpoint as well.

North of the city of Aleppo, meanwhile, government forces killed at least 21 rebels in the Handarat area as the military forged ahead with its efforts to try to sever opposition supply lines into the divided city. Aleppo has been carved into government- and opposition-held sectors since mid-2012.

Syria's SANA state news agency said the military took control of territory in the Mallah farms near Handarat after killing a "large number" of terrorists -- the term the government uses for those opposing its rule.

The Observatory's Abdurrahman warned that government forces have put pressure on rebels in the Handarat area, and are inching toward a complete siege of rebel-held areas of Aleppo.