Fierce clashes pitted Syrian rebels against troops and fighters loyal to the Lebanese Shiite movement Hezbollah in several villages near Qusayr on the border with Lebanon on Tuesday, a watchdog said.
Meanwhile, a military source told AFP the army expects to seize the town of Qusayr, a rebel stronghold, "within days."
"The army is leading the campaign on the northern and eastern fronts, and Hezbollah is leading the fight on the southern and western fronts," said Syrian Observatory for Human Rights director Rami Abdel Rahman.
"The army is advancing in the Qusayr region, and the capture of the city is just days away, at most," the military source said on condition of anonymity.
"The aim is to cleanse the region of terrorists in order to guarantee the safe return of residents" who fled fighting in the area, the source added, using the regime term for rebels.
But Abdel Rahman said the rebels' "fierce resistance" to the onslaught would make the army's advance difficult.
Rebel "morale is high. They are willing to fight to the death to defend their villages and their city," he said.
The Observatory and the opposition say the army is backed by elite fighters from Hezbollah, a staunch backer of President Bashar al-Assad's regime.
Hezbollah says those fighting are Lebanese party members who have lived in Syrian border villages for decades and are defending themselves against "rebel attacks".
On Monday, a Hezbollah official described the group's actions as "a national and moral duty in the defence of the Lebanese in border villages".
Hezbollah's involvement in Syria's spiralling conflict has been condemned by the Syrian opposition, which views it as a "declaration of war," and by Lebanese opponents of the Shiite movement, who fear Lebanon may be dragged into Syria's conflict.
For more than a week, rebels have fired shells hitting the eastern Lebanese Hermel region, a Hezbollah bastion, in what they say is retaliation for strikes by the group.
On Tuesday, two new mortar rounds hit the city of Hermel, causing light damage to a house.