The U.N. Children's agency says airstrikes in Syria's rebel-held northern Idlib province a day earlier may be the deadliest attack on a school since the country's war began nearly six years ago, leaving 22 children and six of their teachers killed.

A team of first responders, the Syrian Civil Defense, and the Britain-based Syrian Observatory for Human Rights said Thursday that the Wednesday airstrikes killed at least 35, mostly children, when they struck in the village of Hass around midday. Initially, the estimated death toll was at 22. The airstrikes hit in a residential area housing a school complex, as children gathered outside.

UNICEF Executive Director Anthony Lake called the airstrikes an "outrage." He added if it is determined that the airstrikes were deliberate, "it is a war crime."