A regime helicopter struck twice a mourning tent in the opposition-held Bab al-Nairab district in Aleppo, Ibrahim Abu Leith told Anadolu Agency.
“Twenty people were killed and 30 others injured in the attack,” he said.
According to Abu Leith, women and children were among the fatalities.
He said the second attack took place when the helicopter dropped another barrel bomb on people who rushed to the scene to help the victims of the first attack.
The injured were rushed to field hospitals amid fears that death toll may rise as a number of civilians remain trapped under the debris of their destroyed homes.
In recent weeks, regime helicopters -- along with Russian and Syrian warplanes -- have struck a number of marketplaces, hospitals and civil defense centers in opposition-held parts of war-battered Aleppo city.
Syria has been locked in a vicious civil war since early 2011, when the Bashar al-Assad regime cracked down on pro-democracy protests -- which erupted as part of the "Arab Spring" uprisings -- with unexpected ferocity.
Since then, more than a quarter of a million people have been killed and more than 10 million displaced across the war-battered country, according to the UN.
The Syrian Center for Policy Research, however, put the death toll from the six-year conflict at more than 470,000 people.