This story includes graphic photographs of injured children.

(CNN) Dozens of children, many younger than 15, were killed in a Saudi-led coalition airstrike that hit a school bus in northern Yemen on Thursday, according to the Houthi-controlled Health Ministry.

The children were on a field trip when their bus was struck at a market, the first stop of the day; 50 were killed and 77 injured, according to the ministry.

Most of the children were inside the bus when the airstrike hit, according to a local medic, Yahya al-Hadi.

The International Committee for the Red Cross said a hospital it supports in Saada had received 29 bodies of "mainly children" younger than 15, and 40 injured, including 30 children.

"(The hospital) is very busy. They've been receiving wounded and dead since the morning and it is nonstop ," Red Cross head of communications and spokesperson Mirella Hodeib said.

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