A large number of civilians from Syrian President Bashar al-Assad's Alawite sect are reported to have been killed or hurt in a village in Hama province.
Some opposition activists said as many as 300 people may have died in Aqrab.
They said pro-government militiamen besieged by rebels had blown up a building in the village where they had been holding the civilians hostage, and it had then been bombed by warplanes.
There has been no word so far from the government or state media.
A video posted online by opposition activists purported to show a young Alawite boy who survived the events in Aqrab, which is about 40km (25 miles) south-west of the city of Hama.
Speaking at a makeshift rebel field hospital, he said that 300 Alawite civilians were killed when government forces destroyed a building where pro-government Shabiha militiamen were holding them hostage.
"Shabiha came to us, claiming they were going to protect us from the rebels," the boy said. "They stopped us from leaving the house, and killed my father, my mother and my brother."
Activists said the militia-controlled building was being besieged by the rebel Free Syrian Army (FSA).
The Shabiha, who are almost entirely Alawite, were using the civilians as human shields, according to the activists.
They said village elders were sent to negotiate a release of the hostages and surrender of the militiamen.
But the elders were seized and killed, the militia threw grenades at hostages who were trying to run away, and then blew the building up as they themselves escaped, the activists claimed.
The Syrian Observatory for Human Rights said it believed between 125 and 150 people, almost all of the Alawites, had been killed by gunfire and explosions in Aqrab.
The UK-based activist group had "received a series of contradictory reports on events in the village of Aqrab, which is inhabited by a minority of Alawites", a statement added.
These included claims that there had been explosions outside Alawite-owned homes in the village, and that rebel fighters had attempted to free the captive civilians, triggering several explosions.
A rebel who said he had fought in Aqrab told the Reuters news agency that fighters had surrounded a house with more than 200 people inside because Shabiha were there.
The militiamen had used women and children as human shields and the house had been shelled by government forces, the rebel said.
Alawites, who make up about 10% of the Syrian population, have largely stayed loyal to President Assad. Many occupy key positions in the government and security forces.
The BBC's Jim Muir in Beirut says it is impossible to verify this complex narrative, and there has been no word at all so far from the government, which is accused of killing its own supporters in order to blame the rebels.
The activists' account even said that government forces later fired rockets at and bombed the building with aircraft, killing more than 200 women and children trapped there, our correspondent adds.
Heavy fighting was also reported, as government forces tried to attack the village from two directions.
The village is not far from Houla, an area where 108 people, including 49 children and 34 women, were killed in May in a massacre which the UN suspects was carried out by Shabiha militiamen, despite denials from the government in Damascus.