At least 14 people have been killed in two separate shellings near the Syrian capital Damascus and the northern city of Aleppo.
In one incident, the Syrian Observatory for Human Rights (SOHR) said, a tank shell hit a bus in the Tariq al-Bab neighbourhood of Aleppo on Tuesday, killing 10 people, including two children.
The Britain-based watchdog group said a woman was also among those killed in the blast, adding that the toll could rise because a number of people had been seriously wounded.
Meanwhile, four people were killed in a separate attack in the eastern countryside of Damascus, sources told Al Jazeera.
The shelling, which hit a crowded market in the rebel-held town of Al Mleha, also injured dozens.
Back in Aleppo, a video filmed by activists and uploaded by the SOHR on YouTube shows chaotic scenes, with the back of a red public bus ablaze and spewing thick plumes of black smoke.
Round loaves of bread apparently sent flying from a street stall litter the ground along with rubble from a nearby building and glass from the windows of the bus and a minivan partially destroyed in the blast.
Groups of men carry injured passengers away, some trailing blood on the ground as they are moved.
Others lift a body that appeared to be partly burnt in the blast, missing its legs.
More than 126,000 people have been killed in Syria since the beginning of the conflict in the country in March 2011.