Two-thirds of the deaths, or 203 people, were in the northern city of Aleppo where the so-called “hell cannons” have been fired on government-held districts of Syria’s second city.
The Britain-based Syrian Observatory for Human Rights, which tracks the violence using sources on both sides, said that 42 children and 25 women were among the dead in Aleppo.
It said more than 700 people had also been wounded during that time.
Syria’s official news agency SANA said on Thursday that “terrorists” fired 11 of the improvised bombs in the southern city of Deraa, wounding several civilians.
The canisters are packed with explosives, fitted with a guide fin and fired by large cannons.
Some 200,000 people have died, the United Nations says.
Chemical weapons have been used, the international chemical weapon watchdog says, and the United Nations says that president Bashar al-Assad’s forces have dropped improvised and indiscriminate barrel bombs on Aleppo.
Meanwhile, rebels in southern Syria say they have taken a step toward unity that may attract more support from their Western and Arab backers, forging a joint defense pact to help shield them from government forces and Islamic State.