The Islamic State killed 270 soldiers, guards and staff when they captured a Syrian gas field on Thursday in the bloodiest clashes between the group and President Bashar al-Assad's forces.
The anti-Assad Syrian Observatory for Human Rights reported on Thursday that 90 people had been killed and that many were missing after the attack against the Sha'ar gas field east of Homs in central Syria.
The Observatory, which monitors violence in Syria through a network of sources in the country on both sides, quoted "trusted sources" as saying that the Islamic State had "killed and executed" 270 people during the assault.
It said at least 40 Islamic State fighters were killed in the offensive.
Islamic State has made rapid gains in Syria, mostly by seizing territory from rival rebel groups.
It used weaponry brought in from Iraq where last month it managed to take large areas from government forces.
It was not immediately possible to verify the report. Syrian state media made no mention of the attack.
About 30 people had managed to escape to the nearby Hajjar field, the Observatory report added.
Activists say the Syrian air force has in recent weeks stepped up attacks on positions held by Islamic State, formerly known as the Islamic State in Iraq and the Levant.
Islamic State has previously taken control of oilfields in Iraq as well as in Syria's eastern Deir al-Zor province.
The group was once the Iraqi affiliate of al-Qaeda, but al-Qaeda disowned it in February after tensions mounted over its expansion into Syria.
Islamic State has declared a 'caliphate' in the areas where it operates in Iraq and Syria, which include the Syrian city of Raqqa as well as Iraq's Mosul.
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 multifaceted civil war after a government crackdown.