Shells struck the city of Homs, the suburbs of Damascus and at least two other smaller towns, the Syrian Network for Human Rights said in a written statement.
The London-based rights group said 173 people, including 23 children and nine women were killed across Syria Sunday. Sixty-four died in Damascus and surrounding countryside, 41 in Aleppo, 26 in Homs, 12 in Hama, 10 in Idlib and the rest in five other cities.
The government reported a large explosion in Homs left at least 15 people dead and 24 wounded, Russia's RIA Novosti news agency said. At least one apartment building was described as heavily damaged by the state-run Syrian Arab News Agency.
China's Xinhua News Agency said the government later said the explosion was caused by a car bomb and that government troops had been doing battle with militant fighters connected to al-Qaida.
The opposition SNHR said the Syrian army has been on the offensive in the suburbs of the capital. Artillery of all sizes has been used and a "large amount of destruction" has resulted.
The SNHR also said many children had been caught up in the violence and medical facilities were being strained. Foreign journalists are prohibited from entering Syria so the reports from both sides could not be independently confirmed.