At least 14 people have been killed after air strikes on rebel-held parts of Aleppo, while six other people including children died when a school was shelled in southern Syria.
Shelling was also reported in the capital, Damascus, with mortar shells fired by opposition groups landing in the residential Qasaa district and close to the Umayyad Mosque, wounding an unspecified number of people.
Fighting on a number of fronts across the country has intensified in recent weeks following the collapse of a US and Russian-brokered ceasefire.
The northern city of Aleppo has seen particularly fierce fighting, as pro-government forces try to capture areas in besieged opposition-held parts of the city.
Bunker-busting bombs were also reportedly used in an attack in Bustan al-Qasr in rebel-held eastern Aleppo.
The opposition-held part of Aleppo has been battered by an intensive aerial campaign since last month, when the truce collapsed after just a week. Syrian pro-government forces are also conducting a ground offensive into the rebel-held districts, advancing slowly in the north, east and south of the city.
The UK-based Syrian Observatory for Human Rights said the air strikes on Aleppo killed at least 16 people, while the Local Coordination Committees said 14 were killed.
In the southern city of Daraa, where the conflict began following anti-government protests in March 2011, rebels fired rockets at government-held areas.
One hit a primary school, killing six people – among them five children – and wounding 18 students, according to state-run news agency SANA.
Some of the wounded are said to be in a serious condition.