The clashes are part of a major escalation of fighting between the al Qaeda offshoot and the military.
The hardline Sunni militants have gained ground in Syria over the past five weeks, bolstered by equipment seized in a lightning offensive last month in neighbouring Iraq.
On Thursday, the group seized the Sha'ar oilfield, east of the central city of Homs, in what the Syrian Observatory for Human Rights said was one of its bloodiest clashes with President Bashar al Assad's forces.
Yesterday, the death toll from the raid rose to 115, the Observatory said. The fate of 250 others was unknown.
A video posted online on Thursday, purportedly of the battle site, showed armed men pacing through a barren desert space speaking in Arabic and German as they reviewed what appeared to be more than 50 bodies.