Syrian rebel forces were bracing Friday, July 27 for the "mother of all" battles in Aleppo, as Washington warned the army could be preparing to carry out a massacre in the country's second city, AFP reports.

Waves of troop reinforcements have been pouring into the northern city - Syria's commercial capital - and a security source told AFP the offensive feared by the rebels could come as early as Friday.

"The special forces were deployed on Wednesday and Thursday on the edges of the city, and more troops have arrived to take part in a generalized counter-offensive on Friday or Saturday," the security source said.

Early Friday, helicopter gunships strafed a string of rebel neighborhoods in the southwest of the city. Clashes also broke out in the Jamiliya district, adjacent to Aleppo's historic old quarter, a human rights watchdog said.

Three people were killed in shelling of the southern Fardoss district and one was shot dead in the Maysaloon neighborhood, the Britain-based Syrian Observatory for Human Rights reported.

In Salaheddin, a rebel bastion in the southwest of the city, hundreds of opposition fighters were bracing for the threatened counter-offensive by the embattled regime of President Bashar al-Assad.