More than 40 people have been killed in northern Yemen after an airstrike hit a bus carrying children.
The attack was carried out by the Saudi-led coalition on the Houthi-rebel controlled area of Dahyan.
The rebels' health ministry said 43 were dead, 29 of them children. More than 60 are being treated for injuries.
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"Today should be the day the world wakes up to the atrocities going on in Yemen," said International Rescue Committe Yemen director Frank McManus.
"A bus full of school children cannot be viewed as mere collateral damage. Even wars have rules, but rules without consequences mean nothing. If there is any chance of innocent lives, especially those of children, being lost in an attack, that attack should not take place."
"Does the world really need more innocent children's lives to stop the cruel war on children in Yemen?" said Unicef regional director Geert Cappelare.
Save the Children said the incident was "horrific".
A spokesman for the Saudi-led coalition said the attack was "legitimate" because it targeted people "planning and targeting civilians", and accused the rebels of using children as "tools and covers for their terrorist acts".
Yemen has been in a brutal civil war since 2015.