Elite fighters from the Lebanese Shiite group Hezbollah, a staunch ally of the Syrian regime, are leading the fight against rebels in the region of Qusayr in the central province of Homs, a watchdog said on Monday.
"It's Hezbollah that is leading the battle in Qusayr, with its elite forces," Rami Abdel Rahman, director of the Syrian Observatory for Human Rights, told AFP.
"It's not necessarily fighters coming from Lebanon. It's Hezbollah fighters from Shiite villages on the Syrian side which are inhabited by Lebanese," he said.
Over the weekend, Syrian regime forces retook control of a string of strategic villages in the region, which is along the border with Lebanon.
That raised fears among rebels that the town of Qusayr itself, a stronghold of the uprising, could fall into government hands.
The Observatory said fighting was raging Monday morning between rebels and Hezbollah fighters around several other villages in the area, leaving two rebels dead.
The area is of key strategic importance because it runs along the border with Lebanon and is near the route running from Damascus to the coast.
On Sunday, the main opposition National Coalition urged Hezbollah to "immediately withdraw its forces from Syrian territory", adding its involvement in Syria's conflict "could drag Lebanon and the region into an open-ended conflict with disastrous consequences".
In the past, Hezbollah has insisted that its members fighting in Homs province were Shiite residents of Syrian border towns engaged in self-defence against rebel forces. The group has not commented on the intensified fighting near Qusayr.
Fighting in the area has spilled over into Lebanon, with rebels reportedly targeting border towns inside Lebanon in response to Hezbollah involvement in the conflict.
Elsewhere in Syria, the Observatory said eight regime security forces were killed when rebels, including from the jihadist Nusra front, detonated a car and fuel tanks at a government checkpoint and security outpost in Damascus province.
On Sunday, 116 people were killed throughout the country, including 39 civilians, 46 rebel fighters and 31 regime forces, said the Observatory, which relies for its information on a network of activists and medical staff on the ground.