BEIRUT (AP) — The U.N. Children's agency called the airstrikes in Syria's rebel-held northern Idlib province a day earlier an "outrage", suggesting it may be the deadliest attack on a school since the country's war began nearly six years ago. The attack, according to UNICEF, killed 22 children and six of their teachers.

A series of airstrikes in the village of Hass around midday Wednesday hit a residential compound that houses a school complex as children gathered outside. The Syrian Civil Defense first responder team and the Britain-based Syrian Observatory for Human Rights said Thursday the airstrikes killed at least 35, most of them children. Initially, the estimated death toll was 22.

The civil defense said there were two schools in the area where the airstrikes hit around midday.

UNICEF Executive Director Anthony Lake called the airstrikes an "outrage." He added if found to be deliberate, the attacks would be considered a war crime.

"This latest atrocity may be the deadliest attack on a school since the war began more than five years ago," Lake said in a statement. "When will the world's revulsion at such barbarity be matched by insistence that this must stop?"

Idlib is the main Syrian opposition stronghold, though radical groups also have a large presence there. It has regularly been hit by Syrian and Russian warplanes as well as the U.S.-led coalition targeting Islamic State militants. An activist at the scene said as many as 10 airstrikes were believed to have hit the residential area Wednesday.

UNICEF said since the beginning of 2016, it has verified at least 38 attacks on schools around Syria, whether in government-held areas or rebel-controlled territory. Before Wednesday's attack, 32 children were killed in 2016 in attacks on schools, according to Juliette Touma, regional UNICEF chief of communication.

A total of 60 attacks were recorded on schools in 2015.

Touma said 591 children were killed in 2015 as a result of the ongoing conflict in Syria, including in attacks on schools.

"In general there are one in three schools in Syria that can't be used anymore because they were damaged or destroyed or used for military purposes or sheltering the displaced," she told The Associated Press, speaking from Amman, Jordan.