"Dozens" of civilians and rebels were killed in a neighbourhood of the central city of Hama which is under siege by regime troops, the Syrian Observatory for Human Rights and activists said Friday.
The Britain-based watchdog, which did not give more precise figures, said it was hard to establish what had happened on Thursday in the Al-Arbaeen district of Hama because telephone lines had been cut.
The Local Coordination Committees (LCC) group, which organises protests on the ground, spoke of a "massacre," saying that about 50 people had been killed, including women and children.
It was not possible to verify the figure from an independent source.
The LCC said buildings in Al-Arbaeen had been damaged and army snipers were deployed on rooftops in the district.
The main opposition Syrian National Council grouping reported that opposition hub Hama had come under "savage bombardment" at dawn on Thursday by regime forces using artillery and helicopter gunships.
"The number of martyrs and wounded is not known as bodies were left lying in the streets, regime forces preventing residents from helping the wounded and burying the dead," the SNC said.