Syrian government forces and Hezbollah launched a fierce campaign to seize more rebel territory in Qusair on Saturday, sources on both sides of the conflict said.
Rebels fighting to topple Assad said additional tanks and artillery had been deployed around opposition-held territory in the border town.
Syrian rebels had said that whatever the outcome, they will plot sectarian revenge attacks on Shi'ite and Alawite villages on either side of the border.
The Britain-based Syrian Observatory, which has a network of activists across Syria, said Assad forces led by Hezbollah were trying to advance from three directions in the city."Every area they didn't have a foothold in, they are trying to gain one now," Rami Abdelraham, head of the Observatory, told Reuters by telephone.
Rebels from across Syria say they have sent some of their units into Qusair.