A Saudi-led coalition battling in Yemen said it carried out a deadly attack in the rebel-held north on Thursday, which the Red Cross said hit a bus carrying children.
In a statement carried by the official Saudi Press Agency, the coalition called the strike a "legitimate military action" that targeted elements responsible for a rebel missile attack on the Saudi city of Jizan on Wednesday.
The coalition did not specify the target, but the International Committee of the Red Cross said the attack hit a bus filled with children, causing dozens of casualties.
"Following an attack this morning on a bus driving children in Dahyan Market, northern Saada, (an ICRC-supported) hospital has received dozens of dead and wounded," the organisation said on Twitter without giving more details.
"Under international humanitarian law, civilians must be protected during conflict," it added.
The Huthi rebels' Al-Masirah TV reported that 39 people had been killed and 51 wounded, "mostly children".
The UN children's agency UNICEF said it was "very concerned with the initial reports of children being killed".
The coalition, which also includes the United Arab Emirates, intervened in 2015 to try to restore the internationally recognised government after the rebels drove it out of the capital Sanaa.
Saudi Arabia shot down a missile fired by the Huthis on Wednesday, with debris killing a Yemeni man and wounding 11 others, the coalition said.
The missile was fired from the rebel-held Yemeni province of Amran towards Jizan, the coalition said.
Last Thursday, attacks on a hospital and a fish market in the strategic rebel-held port city of Hodeida killed at least 55 civilians and wounded 170, according to the ICRC.
The coalition denied responsibility for those attacks.