A record 566 dead bodies were found in Syria on Sunday after six days of fighting.
The total is the highest number of victims found in one day since the war began in March 2011, according to opposition group Local Coordination Committees in Syria (LCC) spokeswoman Rafif Jouejati.
At least 450 of the bodies were found in Damascus suburb Jadidat al-Fadel.
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The past six days saw some 3,000 members of security forces attack the area, with at least 300 of the dead known to be civilians and 150 members of the rebel Free Syrian Army, reports said.
YNet reported another 85 bodies were discovered in a second suburb, Jdeidet al-Fadel, where a days-long battle among rebels and government forces left mostly women and children dead. Some activists put the death toll as high as 250.
"The Local Coordination Committees in Syria extends its deepest condolences to the families of the martyrs of our revolution for freedom and dignity in general," the group said in a statement.
"The LCC holds the Syrian regime fully responsible for the brutal crimes committed against unarmed civilians, and calls on the United Nations Security Council to refer these crimes to the International Criminal Court as war crimes."
The the pro-government Syrian Arab News Agency, however, did not acknowledge civilian deaths, saying that armed forces "inflicted heavy losses upon terrorists in a special operations [sic] carried out against their dens and gatherings in al-Janoudia, al-Hamama, al-Yaqoupia, Katrun, al-Shughr in Jisr al-Shughour countryside, inflicting heavy losses upon terrorists and their equipment."