(CNN) -- At least 15 boys were killed when Syrian forces dropped a barrel bomb on a mosque operating as a makeshift school in Aleppo, the opposition Syrian Coalition said Wednesday.
Barrel bombs are drums packed with explosives and shrapnel, which are then dropped from the sky.
Tuesday's attack is the second barrel bomb raid in two days in the northern Syrian city, which has become a flashpoint of the country's civil war.
"Each and every day that the barrel-bombing of Aleppo continues, the Assad regime reminds the world of its true colors," U.S. Secretary of State John Kerry said in a statement after the attack.
"Each and every barrel bomb filled with metal shrapnel and fuel launched against innocent Syrians underscores the barbarity of a regime that has turned its country into a super magnet for terror."
Later, Kerry defended U.S. policy toward Syria in an exclusive interview on CNN's "The Lead with Jake Tapper."
However, he did not dispute entirely a U.S. intelligence assessment that Syrian President Bashar al-Assad has regained some footing since agreeing to turn over chemical weapons to the international community to avoid possible military action against his regime.
"It's fair to say that Assad has improved his position a little bit, yes," Kerry said. "But he's still not winning. This is a stalemate."
Barrel bomb toll
On Monday, 26 people were killed, the UK-based Syrian Observatory for Human Rights said. And over the weekend, barrel bombs killed 126 people there.
"The mosque was being used as a school after all schools in the area were either hit or brimming with internally displaced civilians," Khalid Saleh of the Syrian Coalition said about Tuesday's attack.
"Giving up on learning was not an option for these children, but the hatred of the Assad regime was blind enough to deprive them not only from their right to education but also from their right to live."
'Children, there are children'
Opposition groups have blamed the attacks on al-Assad. The Syrian government has previously said that military operations are targeting "terrorist groups" in neighborhoods of the city.
Videos posted online after the attack on the Othman Bin Affan mosque Tuesday showed angry residents, arms raised to the sky, cursing at al-Assad.
"Children, there are children," some shouted.
One man, standing in front of a child's body, sobbed, "Oh God, we've had enough."
"Why us," a child told an activist who filmed the aftermath of the attack. "We are not rebel fighters, we are children."
CNN cannot independently verify the death tolls or the authenticity of the videos.
The United Nations says more than 100,000 people have been killed in Syria since fighting began in 2011.