BEIRUT — 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 Observatory put the death toll among children at 16 children and five women. It was not immediately possible to reconcile the two figures, but divergent death tolls are not uncommon in a conflict-torn Syria that has been largely inaccessible to international media for over two years.

UNICEF and the Civil Defense said the death toll is likely to rise, as rescue efforts continue. The civil defense said there were two schools in the area hit with 11 airstrikes 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 militant 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.

Children are bearing the brunt of the violence that has engulfed Syria.

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

Wednesday’s attack was the deadliest attack on schools in 2016, according to Touma. Before Wednesday’s attack, the deadliest attack on a school was reported in April 2014, when 30 children were killed when airstrikes hit a school in the rebel-held part of Aleppo city, according to UNICEF.