The Head of the UN Children’s Fund (UNICEF) has strongly condemned the airstrike against a school bus in northern Yemen on Thursday, which according to news reports has killed more than two dozen children.

“Attacks on children are absolutely unacceptable,” said UNICEF Executive Director Henrietta Fore on Twitter on Thursday. “I’m horrified by the reported airstrike on innocent children, some with UNICEF backpacks. Enough is enough.”

The attack hit the schoolbus in a market in Dahyan, in the northern province of Saada. While the exact death toll remains to be confirmed, initial news reports indicate that the number of casualties could be well above 60, with dozens severely injured. Many of the children were reported to be under the age of 10.

“Attacking children is the lowest any party of this conflict can go,” UNICEF Yemen Resident Representative Meritxell Relaño told UN News. “There is no justification whatsoever to attacking children.”

According to the UN Children’s Fund, since conflict between pro-Government forces and Houthi rebels escalated in 2015, about 2,500 children have been killed and 3,600 maimed in Yemen.

The head of the UN agency there called all warring parties to “respect international humanitarian law,” and spare children, civilians and civilian infrastructure to prevent Yemen from falling “further into the abyss and the humanitarian catastrophe” it has been facing for over three years.

Yemen’s conflict has its roots in uprisings that date back to 2011, but fighting escalated in March 2015, when an international coalition led by Saudi Arabia intervened militarily at the request of Yemen’s President.

Attacks against civilians have been the scourge of this conflict. According to the UN Office for the Coordination of Humanitarian Affairs (OCHA), “tens of civilians were killed in violence in several governorates” in the past 10 days alone. On 3 August, during a particularly deadly attack, one of the last functioning hospitals, Al Thawra in Al Hudaydah, was struck, reportedly causing the death of dozens of vulnerable, sick and injured civilians.

“We’ve said this before and we are saying it again - parties to the conflict are obliged to do everything possible to protect civilians and civilian infrastructure. This is not a voluntary commitment - it is mandatory on all belligerents,” said Lise Grande, UN Humanitarian Coordinator in Yemen, in the latest OCHA report on the situation there. “So many people have died in Yemen - this conflict has to stop.”