UNITED NATIONS, Aug. 9 (Xinhua) -- The chief of the UN Children's Fund (UNICEF) strongly condemned the airstrike against a school bus in northern Yemen on Thursday, which reportedly killed more than two dozen children.

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

The attack hit the school bus in a market in Dahyan, in the northern province of Saada. While the exact death toll remains to be confirmed, initial reports said 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, according to UN News.

According to the UNICEF, 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.

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 its government.

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.