At least 60 soldiers have died as the army battles to retake a gas field in central Syria where jihadists killed 270 people, a pro-regime newspaper reported on Sunday.
Al-Watan, citing a military source, give a toll of "60 martyrs" for a counter-offensive against the Islamic State (IS) group around Shaar gas field that a security official said was launched Friday.
The Syrian Observatory for Human Rights also reported "violent clashes... in and around the gas field".
On Saturday, the Observatory said it had documented "the deaths of 270 people killed in fighting or executed" by IS militants.
"A large majority of the men killed were executed at gunpoint after being taken prisoner" in the seizure of the field, said Observatory chief Rami Abdel Rahman.
"Eleven of the dead were civilian employees, while the rest were security guards and National Defence Forces members," he added.
The Britain-based Observatory, which relies on a network of activists and medics on the ground, said at least 40 IS militants were killed in the counter-attack, which was backed by aircraft.
Thursday's takeover of the Shaar field was the "biggest anti-regime operation by the IS" since the jihadist group rose to prominence last year among rebel groups in Syria's conflict, it said.
On Sunday, the Observatory said the IS had carried out another 24 executions at a Deir Ezzor oilfield in eastern Syria of men abducted during the past month.
At least eight of the men killed on Saturday night were fighters from other anti-regime rebels groups, it said.
The Observatory also reported 17 rebels killed in clashes with the IS near rebel-held Akhtarin, a town in the northern province of Aleppo.
IS jihadists proclaimed an Islamic "caliphate" straddling Syria and Iraq last month and have also taken over oil-rich Deir Ezzor province.
Deir Ezzor borders Homs province as well as Iraq, where the jihadist group has spearheaded a major Sunni militant offensive that has seen large swathes of territory fall out of Baghdad's control.