The Islamic State group killed more than 50 people in an attack on two government-held villages in central Syria's Hama province on Thursday, a monitor said.
The jihadists executed some of those killed in the assault, which comes as IS loses territory elsewhere in Syria and in neighbouring Iraq.
The Britain-based Syrian Observatory for Human Rights said the attack on the villages of Aqareb and Al-Mabujeh killed at least 15 civilians and 27 pro-government fighters.
It said another 10 bodies were yet to be identified as government fighters or local residents.
IS also lost 15 fighters in the dawn attack on the two villages in the east of the province, the monitor said.
Observatory director Rami Abdel Rahman said at least three of the civilians were killed execution style with knives, a man and his two children.
It was unclear if others had also been killed the same way or died in fighting.
Syria's state news agency also reported 52 people had been killed in the IS assault, though it described the attack as against Aqareb only.
It said IS fighters had mutilated the bodies of villagers and looted their homes, and said that 15 of the dead were children.
The Observatory said IS had seized control of Aqareb and part of Al-Mabujeh in the attack, which began with heavy shelling on a nearby regime checkpoint.
"Despite the arrival of reinforcements, government forces have been unable to repel the attack so far," Abdel Rahman told AFP.
He said the fighting was continuing and had spread to the outskirts of nearby villages.
IS has attacked Al-Mabujeh before with devastating effect.
In March 2015, it executed at least 37 civilians in the village, whose population includes Sunni Muslims as well as those from the Ismaili sect and the Alawite community to which President Bashar al-Assad belongs.
During the same assault the group kidnapped at least 50 civilians, half of them women.
Control of central Hama province is divided between Syria's government, rebel forces and IS, which is present mostly in the east of the area.
The attack comes as IS faces pressure in the north of the country, where an alliance of Kurdish and Arab fighters is preparing an assault on its bastion Raqa city.
The jihadist group has also lost most of the Iraqi second city of Mosul, which it captured in 2014.
The battle for Raqa has stirred tensions with neighbouring Turkey, which is fiercely opposed to the Kurdish fighters now leading the operation to capture the city.
On Thursday, Turkish Foreign Minister Mevlut Cavusoglu called for the removal of US diplomat Brett McGurk, who coordinates the US-led coalition against IS, saying he was supporting the Kurdish fighters.
"It would be useful if this person was replaced," Cavusoglu said.
More than 320,000 people have been killed in Syria since the conflict began with anti-government demonstrations in 2011.
The conflict has proved stubbornly resistant to international efforts to find a political solution, but a new round of UN-sponsored talks are underway this week in Geneva.
The UN-sponsored process has been somewhat overshadowed by a parallel track convened in Kazakh capital Astana by regime supporters Russia and Iran along with rebel backer Turkey.
The three nations agreed earlier this month to establish four "de-escalation zones" in Syria, and are now hammering out the details of the plan.
In the meanwhile, Syrian government and opposition delegations in Geneva agreed Thursday to informal discussions on constitutional issues.
A new constitution is one of four separate "baskets" on the agenda at the talks, along with governance, elections and combating "terrorism".
Government lead negotiator Bashar al-Jaafari said none of the topics had yet been discussed, three days into the meeting.
And he played down the plan to discuss constitutional issues.
"The work of these experts will have nothing to do with the constitution... They will not take decisions," he said.