(CNN) -- At least 83 people died in aerial bombing conducted by government forces in Aleppo, Syria, said the Local Coordination Committees, an opposition group.
At least 27 children and eight women were among the dead, the group said; it reported that 50 more people had sustained injuries in Sunday's attacks.
The United Nations Children's Fund said in a statement that it was "horrified" by reports that improvised bombs dropped from helicopters killed between 14 and 28 children.
"It is absolutely unacceptable for children to be targeted in this manner, whether through the use of indiscriminate weapons resulting in mass casualties or by any other means," the statement said.
Helicopters dropped barrel bombs on 12 neighborhoods in the city, where people have strongly backed rebels opposing the government of President Bashar al-Assad.
The opposition organization says it is not the first time the Syrian regime has dropped barrels filled with explosives on Aleppo neighborhoods.
The LCC said 135 people died overall in the conflict on Sunday.
The United Nations estimates that more than 100,000 people have died since the Syrian conflict began in March 2011. Hundreds of thousands of civilians have also been injured.
It began when government forces cracked down on peaceful protesters during the Arab Spring movement, and slowly spiraled into civil war.
More than 2.3 million people have fled Syria and another 4.25 million Syrians have become refugees within their country since the civil war began, according to an Amnesty International report released on Friday.
CNN's Holly Yan contributed to this report