The main bloc of the Syrian opposition on Sunday accused staunch Damascus ally Hezbollah of "militarily intervention" in Syria, calling it a danger to regional peace and security.
The Syrian National Council (SNC) said members of the powerful armed movement on Saturday attacked "three Syrian villages in the Qusayr region near the Lebanese border."
The operation in Homs province led not only to "civilian casualties and the exodus of hundreds of people," but also "stoked sectarian tensions" in the area, the SNC said in a statement.
Lebanon's Hezbollah is a Shiite party, while most of the Syrian population and the rebels battling Syrian President Bashar al-Assad's regime are Sunni.
The SNC said Hezbollah was employing "heavy weapons openly and under the auspices of the Syrian regime army."
This is a "serious threat to Syrian-Lebanese relations and regional peace and security," it said, adding that the Beirut government has a responsibility to end this "aggression."
Lebanon was dominated politically and militarily by its larger neighbour Syria for three decades, and the small Mediterranean country is sharply divided over the Syrian revolt which erupted in March 2011.
Hezbollah and its allies in the ruling coalition back Assad's regime, but the Sunni-led March 14 movement and its allies support the rebellion.
The Shiite party, Lebanon's most powerful military force, occasionally announces the death of one of its fighters killed "carrying out his jihadist duty," but without clarification.
In October 2012, Hezbollah leader Hassan Nasrallah acknowledged that party members had fought Syrian rebels, but said they were acting as individuals and not under the party's direction.
On Saturday, the Local Coordination Committees -- a grassroots network of activists on the ground in Syria -- reported fighting between rebels and members of Hezbollah around Qusayr.
It said they were trying to break into the rebel-held city, which has been under daily army bombardment for the past year.