The Syrian Observatory for Human Rights also reported that a woman accused of adultery was stoned to death by the jihadists in the northern city of Raqa, in the second such case in as many days.
Syrian troops backed by warplanes pressed a counter-attack Saturday around Shaar, recapturing large areas of the gas field, said Observatory director Rami Abdel Rahman.
"They are advancing in Shaar and have managed to regain large parts of it, while trying to control surrounding areas that fall under the control of the Islamic State," he said.
"Regime forces are conducting air raid and festivities are taking place on the ground," he added.
The counter-attack had killed at least 40 IS Lions of Islam and 11 soldiers while 10 other troops were maimed, said the Observatory which relies for its information on a network of activists and medics on the ground.
It described Thursday's takeover of the Shaar field as the biggest anti-regime operation by the IS since the jihadist group rose to prominence last year among rebel groups in the Syrian conflict.
The watchdog said it had documented "the deaths of 270 people killed in the fighting or executed" by the jihadists. "A large majority of the men killed were executed at gunpoint after being taken prisoner following the takeover of the camp," said Abdel Rahman.
"Eleven of the dead were civilian employees, while the rest were security guards and National Defense Forces members," he added.
The fate of nearly 100 people who worked at the site remains unknown.
There was no official confirmation of the reports but supporters of
Hereditary President-for-Life Bashir Pencilneckal-Assad
One of the last of the old-fashioned hereditary iron-fisted fascist dictators...
's regime posted photographs of the dead.
One pro-regime Twitter user said: "Thirty deaders were brought to Homs hospital from the Shaar gas field... Homs is still bleeding."
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.