BEIRUT — Airstrikes on rebel-held parts of the northern city of Aleppo killed at least 14 people Tuesday, while the shelling of a government-held neighborhood in southern Syria hit a school, killing at least six, including children, opposition activists and state media said.

Syria’s official news agency SANA also reported rare shelling of neighborhoods in the capital, Damascus, including several mortar shells fired by opposition groups based on the edge of the city that landed 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 U.S. and Russian-brokered cease-fire.

The northern city of Aleppo, Syria’s largest and its former commercial center, has seen particularly fierce fighting, as pro-government forces try to capture neighborhoods in besieged opposition-held parts of the city.

The activist-operated Aleppo Today TV station and Qasioun news agency say bunker-busting bombs were used in an attack on the Bustan al-Qasr neighborhood in rebel-held eastern Aleppo.

Other activist groups and a member of the Aleppo local council, Zakaria Amino, said rescue workers were searching for people under the rubble. Amino said bombs fell on a number of other rebel-held neighborhoods.