Dozens of children, many believed to be under the age of 10 and on their way to summer camps, have been killed after a Saudi-led coalition airstrike hit a school bus in northern Yemen on Thursday.

The bus was struck as it was driving through a market in the rebel-held province of Saada, according to the Houthi-run Al-Masirah TV.

At least 43 people were killed and 63 injured in the strike, according to the Houthi-held health ministry.

The International Committee for the Red Cross (ICRC) said that a hospital it supports in Saada had received 29 dead bodies of “mainly children” under 15 years of age, and 40 injured, including 30 children.

“(The hospital) is very busy. They’ve been receiving wounded and dead since the morning and it is non-stop ,” ICRC head of communications and spokesperson Mirella Hodeib said.

The Saudi-led coalition called the airstrike a “legitimate military operation,” and a retaliation to a Houthi ballistic missile that targeted the kingdom’s Jizan province on Wednesday night, according to Saudi Arabia’s official news agency.

“The targeting that happened today in Saada province was legal military action to target elements that planned and executed the targeting of civilians in the city of Jizan last night, killing and wounding civilians,” the Saudi Press Agency cited official coalition spokesperson Turki al-Maliki as saying.