Jihadists have killed 270 Syrian regime fighters, civilian security guards and staff since seizing a gas field in Homs province, most of whom were executed, a monitoring group said Saturday.
The Syrian Observatory for Human Rights also reported that a woman accused of adultery was stoned to death by the Islamic State (IS) in the northern city of Raqa, in the second such case in as many days.
The Observatory 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 death of 270 people killed in the fighting or executed" since the takeover of the field in central Syria.
"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.
On Saturday, regime forces, a day after launching a counter-attack, had "re-taken large areas of Shaar", according to Abdel Rahman. "Fighting is continuing around the gas field".
The counter-attack had left at least 40 IS militants dead, while 11 soldiers had been killed and 10 others who were injured had been taken to hospital in Homs, said the Observatory, which relies for its information on a network of activists and medics on the ground.
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.
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.
On Friday night on a main square in Raqa, IS jihadists stoned to death a woman they accused of adultery, the Observatory said, citing residents of their stronghold.
"Because residents refused to take part in the stoning, the IS fighters themselves executed the woman," it said, adding that they pulled up a lorry filled with rocks for the killing.
The jihadists also stoned a young woman to death for adultery in Raqa on Thursday. Abdel Rahman identified her as a 26-year-old widow.
According to a local activist, who said that the public stoning took place in a square in the Raqa provincial town of Tabqa, residents are "terrified but do not dare react to such barbaric methods".
The IS, 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.