Syrian regime troops killed at least 175 rebel fighters in an ambush on the outskirts of Damascus, Syrian state media said on Wednesday Al Arabiya reported.
According to activists, Lebanon's Shiite militant group Hezbollah led the assault, joining hands with Syria's government forces.
Syrian Arab News Agency (SANA) reported that the major attack, which took place at dawn in the in the opposition-held area of eastern Ghouta, targeted mostly al-Qaeda-linked fighters and other foreign fighters.
Lebanon's al-Manar television, operated by Hezbollah, broadcasted images showing dozens of men's bodies lying on a rural road near Otaiba, a town in the eastern Ghouta area.
SANA said the operation delivered a "smashing blow to terrorists," a term used by Syria's state media in reference to opposition fighters.
The capital's suburbs, including eastern Ghouta, have mostly been under the control of opposition forces since March 2011, the beginning of the uprising against the President Bashar al-Assad.
Britain-based Syrian Observatory for Human Rights also reported the assault, saying that 70 rebels were killed. It added that the numbers of those killed will surge because 89 rebels were reported missing.
A field commander in eastern Ghouta was quoted by SANA as saying that most of the killed rebels near the Oteibah lake belonged to the al-Nusra Front rebel group. SANA posted photographs showing dozens of men's bodies lying in an open field on its website.