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