Syrian regime forces launched an offensive Saturday to retake a gas field in Homs province seized two days ago by jihadists who killed 270 people, most of them executed, a monitoring group said.
The Syrian Observatory for Human Rights also reported that a woman accused of adultery was stoned to death by Islamic State (IS) jihadists in the northern city of Raqa, in the second such case in as many days.
Regime forces 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 clashes are taking place on the ground," he added.
The counter-attack had killed at least 40 IS militants and 11 soldiers while 10 other troops were wounded, 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 Defence 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 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."
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 jihadists proclaimed an Islamic "caliphate" straddling Syria and Iraq last month and have 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.
On Saturday regime forces clashed with IS jihadists in the western part of the province, the Observatory said.
Meanwhile nine people were killed in a car bomb attack in the rebel-held city of Douma, northeast of Damascus, it said.