A strike on a bus in rebel-held northern Yemen killed at least 29 children Thursday, the Red Cross said, as the Saudi-led coalition faced a growing outcry over the attack.
The coalition said it had carried out what it called "legitimate military action" in the area targeting Huthi rebels responsible for a deadly missile attack on the Saudi city of Jizan on Wednesday.
But the International Committee of the Red Cross said the strike hit a bus filled with children in the Huthi stronghold of Saada, causing dozens of casualties.
"A hospital supported by our team in Yemen received the bodies of 29 children under the age of 15 and 48 wounded, including 30 children," the ICRC said on Twitter.
A spokesman for the Red Cross in Sanaa told AFP the toll was not final as casualties from the attack were taken to several hospitals.
The Huthis' Al-Masirah TV, quoting the rebel health ministry, reported that 50 people were killed and 77 wounded, "mostly children", though it was not possible to verify that toll.
International aid agencies denounced the attack and the loss of civilian lives.
"Under international humanitarian law, civilians must be protected during conflict," the ICRC said.
Geert Cappelaere, the UN Children's Fund regional director in the Middle East and North Africa, said all the children on the bus were "reportedly under the age of 15".
"Does the world really need more innocent children's lives to stop the cruel war on children in Yemen?", he added.
The Save the Children charity, quoting its staff, said that at the time of the attack the children were on a bus heading back to school "from a picnic when the driver stopped to get a drink".
"Save the Children condemns this horrific attack and is calling for a full, immediate and independent investigation into this and other recent attacks on civilians and civilian infrastructure," it said.
Doctors Without Borders (MSF) said in a tweet that it was "deeply saddened" by the strike.
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.
"The coalition will take all necessary measures against the terrorist, criminal acts of the Huthi militia, such as recruiting child soldiers, throwing them in battlefields and using them as tools," coalition spokesman Turki al-Maliki said, referring to Thursday's attack.
The Huthis have in recent months ramped up missile attacks against Saudi Arabia, which Riyadh usually says it intercepts.
Wednesday's attack brings the tally to 165 rebel missiles launched since 2015, according to the coalition, which that year joined the Yemeni government's fight against Huthi rebels.
On August 2, 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.
Aid agency CARE International noted that Thursday's air strike came a week after the Hodeida attacks
"This latest air strike, only a week after the attacks on Hodeida city, demonstrates a continued disregard for human life and suffering," said Johan Mooij, the agency's country director in Yemen.
"It is beyond cruel; innocent children's lives have been lost."
The war in the impoverished country has left nearly 10,000 people dead and unleashed what the UN describes as the world's worst humanitarian crisis.
The United Nations envoy to Yemen, Martin Griffiths, told the Security Council last week that "a political solution" to end the war in Yemen was "available" and that the warring sides would be invited to talks on September 6 in Geneva.
UN-brokered negotiations on Yemen broke down in 2016 amid demands for a rebel withdrawal from key cities and power-sharing with the Saudi-backed government.