At least 50 people were killed when the Syrian army attacked suburban areas in the central city of Hama, the National Organization for Human Rights in Syria said Wednesday.
The dead included eight children, the group said without providing further details, DPA reported.
Hama has for weeks been the focal point for protests against the government of President Bashar al-Assad.
Meanwhile, broadcaster Al-Jazeera reported that the Syrian army was conducting raids in Deir al-Zour in north-eastern Syria, making mass arrests in the neighbourhoods of al-Qasur and al-Ardi.
Al-Jazeera quoted unidentified witnesses as saying that columns of tanks were moving into al-Ardi.
Around 34 civilians, including 11 children, were killed Tuesday in Deir al-Zour, the opposition group, the Local Coordination Committees of Syria, said.
Around 1,730 civilians and 406 security personnel have been killed since the anti-government protests started in Syria in mid-March, according to the Syrian Observatory for Human Rights.
It is hard to verify these reports, as the Syrian authorities have barred most foreign media and international human rights groups from the country.