Columns of armed forces and armour have poured into Aleppo over the past two days, with troops firing on a string of rebel neighbourhoods in the battle for control of the pivotal northern city.
Amid widespread expectations that an increasingly desperate President Bashar al-Assad will pull out all the stops to secure Aleppo, residents of some areas of the commercial capital fled in fear of a massacre.
The redeployments were "almost complete," a Syrian security official told AFP, adding however that "rebels are stationed in narrow streets, in which fighting will be difficult."
Colonel Abdel Jabbar al-Okaidi of the rebel Free Syrian Army told AFP via Skype that "we expect a major offensive at any time."
Syrian Observatory for Human Rights director Rami Abdel Rahman said, "The question is, to what extent will regime troops resort to excessive use of force, because if this is the case, there will be hundreds of deaths."
Aleppo has been the scene of fierce fighting since rebels launched a major offensive on Jul 20 from rear bases across the nearby border with Turkey.
The Observatory said helicopter gunships attacked the southwest of the city, in the Salaheddin, Bustan al-Qasr, Sukari, Al-Mashhad and Al-Azamiya neighbourhoods.