Syrian troops and Hizbollah fighters pressed their assault on the strategic town of Qusair on Friday, but the rebels say hundreds of fighters have broken through army lines to join the battle.
Syrian state television reported that the troops have captured a district in the north of Qusair, leaving rebels in the town with little chance to escape.
But the opposition says hundreds of rebel reinforcements have now arrived.
The offensive began on 19 May.
Meanwhile, President Bashar al-Assad said he was "very confident" of victory in the conflict, which began in March 2011.
"There is a world war being waged against Syria and the policy of (anti-Israeli) resistance... (but) we are very confident of victory," he told Hezbollah's Al-Manar television on Thursday.