Opposition activists in the city told the BBC the Syrian army was being bolstered by Hezbollah fighters and militias loyal to President Bashar Assad.
Syrian forces last week retook Qusair and Sunday recaptured villages in the region between the Lebanese border and Homs.
Rebels and a senior commander of the Lebanese resistance movement told Gulf News about 2,000 Hezbollah fighters had massed outside Aleppo Sunday, Gulf News reported.
U.S. officials responded by saying the development could push the Obama administration to arm the rebels, the BBC said.
A State Department spokeswoman said Monday the United States would "continue to look for ways to help the opposition and increase aid."
"The president has talked about how boots on the ground is not an option -- so all options short of that," the spokeswoman said.