"Terrorists are assembling a lot of fighters in Kaft Hamrah. According to our data, they are forming assault groups," the source said.
According to the source, additional units were redeployed from Idlib province to Aleppo city.
"We do not know yet where the extremists plan to strike, but is its very likely that they will try to block Castello road, which is the city's northern supply route," the source added.
On September 9, Russian Foreign Minister Sergey Lavrov and US Secretary of State John Kerry announced a new peace plan to address the conflict in Syria that called for a ceasefire to come into force on Monday.
The next ministerial meeting on Syria is due to be held on the sidelines of the UN General Assembly in New York on September 21 and is expected to become an opportunity for the resumption of the negotiations in Geneva.
Syria has been mired in civil war since 2011, with government forces loyal to Assad fighting numerous opposition factions and extremist groups.