More than 700 people were killed in clashes over two days between pro-government forces and Islamic State of Iraq and the Levant (Isis) militants in Syria.
Activists from the Syrian Observatory for Human Rights (SOHR) said that the deaths represented the bloodiest fighting since the civil war began in 2011.
Forces loyal to President Bashar al-Assad and Isis militants battled following the rebel group's capture of the Shaar gas field east of Homs city.
By Thursday 270 have been killed in clashes while the death toll for the day's combined unrest was 396.
Friday's death toll reached 314 while 90 more people are still unaccounted for. Precise numbers of Isis and pro-government casualties in the overall death toll remain unknown.
Rami Abdul Rahman, head of SOHR, said that the deaths were the first time 700 had been killed in the space of two days since the onset of the conflict.
Rahman said that Isis' attack on the gas field was the "most important so far against the [Syrian] government".
Last month, Isis declared it had formed an Islamic caliphate straddling the Iraqi-Syrian border and appointed its leader Abu Bakr al-Baghdadi as the "caliph" of all the world's Muslims.
While Isis' offensive has been focused on northern Iraq in recents months, the militants continue to hold onto territory in eastern Syria such as Deir Ezzor, seizing all of its oil fields.
The group also continue to crack down on those who fall under its control for activities counter to their radical interpretation of Islam.
Jihadists stoned to death two women accused of adultery in the northern Raqqa province after being sentenced by an Islamic court. A mob reportedly carried out the death sentence in the Al Taqaba market area last week.