An international monitoring group says militants from the Islamic State, formerly known as ISIL, killed as many as 270 soldiers, guards, and staff when they captured a Syrian natural gas field on July 17.
The Britain-based Syrian Observatory for Human Rights said on July 19 that many of those killed in central Syria's Sha'ar gas field to the east of Homs were executed by the militants from the Al-Qaeda splinter group.
It was not immediately possible to verify the report.
Syrian state media had made no mention of the attack through the morning of July 19.
The Observatory says more than 170,000 people have been killed in Syria's conflict, which started as a peaceful protest movement in 2011 but descended into a civil war after a crackdown by troops loyal to Syrian President Bashar al-Assad.