Jihadists killed at least 115 Syrian regime fighters, civilian security guards and employees when they seized a gas field in Homs province, a monitoring group said on Friday, updating an earlier toll.
The Syrian Observatory for Human Rights described Thursday's takeover of the Shaar field as "the biggest" anti-regime operation by the Islamic State (IS) since it emerged in the Syrian conflict last year.
The Observatory also said the fate of another 250 people who worked at the site but went missing during the fighting remains unknown.
"The number of people, most of them security guards and members of the (paramilitary) National Defence Forces, killed in the IS attack on the Shaar gas field in Homs province has risen to at least 115," said Observatory director Rami Abdel Rahman.
Earlier, the watchdog gave a toll of at least 90 killed.
The government did not officially confirm the deaths, but supporters of President Bashar al-Assad's regime posted photographs of the dead, and branded their killings as a "massacre".
"Eleven of the dead were civilian employees, while the rest were security guards and National Defence Forces members," Abdel Rahman said.
The Observatory had initially reported 25 of the killed were civilians, but later revised its toll downwards.
Abdel Rahman also said the men were killed "after they had been taken prisoner", and that "the fate of more than 250 others, civilians and fighters, remains unknown".
One pro-regime Twitter user said: "Thirty martyrs were brought to Homs hospital from the Shaar gas field... Homs is still bleeding."
He also branded the killings as a "massacre", and posted pictures of the dead.
Gruesome footage apparently recorded by the jihadists at the gas field and distributed via YouTube showed dozens of bodies, some of them mutilated, strewn across a desert landscape.
One video shows a jihadist posing with the bodies as he speaks in German interspersed with religious terms in Arabic, seemingly celebrating the killings.
Abdel Rahman meanwhile condemned the deaths.
"The Observatory condemns summary execution as a war crime, regardless of which side it is committed by in the Syrian conflict," he said.
"Summary execution is a war crime - whether of civilians or combatants. They are prisoners of war and must not be executed."
The Islamic State, which proclaimed a "caliphate" straddling Syria and Iraq last month, has also taken over Syria's 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 the Baghdad government's control.