BEIRUT, Lebanon - Jihadists have killed 270 Syrian regime fighters, civilian security guards and employees since seizing a gas field in Homs province, a monitoring group said Friday.
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 watchdog, updating an earlier toll of 115, said it had documented "the death of 270 people killed in the fighting or executed" since Thursday.
"A large majority of the men killed were executed at gunpoint after being taken prisoner following the takeover of the camp," said Observatory director Rami Abdel Rahman.
"Eleven of the dead were civilian employees, while the rest were security guards and National Defence Forces members," he added.
Abdel Rahman said a counter-attack by regime forces on Friday left 40 IS militants dead.
The fate of nearly 100 people who worked at the site remained unknown, according to earlier figures released by the Observatory.
The Syrian 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".
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.