The Syrian military continued its offensive on Qusayr amid reports that at least 28 Hezbollah fighters helping the army in the strategic attack have been killed so far.
Syrian state media said army units pushed deeper into the town near the Lebanese border on Monday, and were fighting street battles with the rebels.
The SANA news agency said President Bashar al-Assad's troops took control of most of the town of Qusayr.
Opposition activists said at least 52 people were killed on Sunday. At least 28 Hezbollah fighters were also killed, they said. Lebanese security officials said funerals for several Hezbollah fighters were held on Monday after the bodies were brought across the border.
Al Jazeera's Rula Amin, reporting from Beirut, said: "It seems the government is able to control most of Qusayr ... it has been a rebel stronghold for about one year."
The attack follows a rare interview with the Assad released on Saturday, in which he said that his government was not using "fighters from outside of Syria, of other nationalities, and needs no support from any Arab or foreign state".
Our correspondent in Beirut said: "Both sides have sought help from other countries.
"Hezbollah is heavily involved in the fighting, and there are hundreds of foreign fighters in Qusayr. The confrontation has taken a rather regional and sectarian tone. This is about Sunnis fighting Alawites and Shia, not just Syrian rebels fighting government forces."
For weeks, fighting has raged around Qusayr, a town in the central province of Homs that has been under rebel control since early last year.
The intensity of the fighting reflects the importance that both sides attach to the area.
By the government's calculations, Qusayr lies along a strategic land corridor linking Damascus with the Mediterranean coast, the heartland of Assad's Alawite sect.
For the rebels, the overwhelmingly Sunni Qusayr has served as a conduit for shipments of weapons and supplies smuggled from Lebanon to opposition fighters inside Syria.
Given the latest battle's location and its inclusion of foreign fighters, fears are growing that sectarian violence in the region will intensify.
At least three people have reportedly been killed and about 40 wounded from two days of fighting between residents of Lebanon's northern city of Tripoli, security sources said on Monday.
There, the largely Sunni neighbourhood of Bab al-Tabbaneh supports Syria's predominantly Sunni rebels, and the adjacent Alawite neighbourhood of Jabal Mohsen supports Assad.
|Assad: To resign is to flee
"Any sectarian conflict in Syria won’t be confined to its borders," said Al Jazeera's Amin. "Both its neighbours, Iraq and Lebanon, are divided along the same sectarian lines ... Both Sunni and Shia militants from both countries are fighting each other now inside Syria."
Earlier in May, Israeli warplanes struck areas in and around the Syrian capital, setting off a series of explosions as they targeted a shipment of highly accurate, Iranian-made guided missiles believed to be on their way to Lebanon's Hezbollah group.
An Israeli army spokeswoman said on Monday that gunfire from Syria hit the Israeli-controlled Golan Heights.
The military said there was no damage caused or injuries.