- Video report by ITV News Correspondent Dan Rivers
At least 29 children were among 50 people killed when a missile hit a bus carrying civilians in Yemen.
It is thought many of the children were on their way to school when the attack happened.
The strike in the Dahyan market in Saada province also injured 77 and was carried out by the Saudi-led coalition fighting Shia rebels, the rebel Health Ministry said.
However, the Saudi-led coalition said it had targeted the rebels, known as Houthis, after they had fired a missile at the kingdom’s south on Wednesday, killing one person.
Al Masirah TV - a Houthi television channel - aired dramatic images of wounded children, their clothes and schoolbags covered with blood as they lay on hospital stretchers.
The International Committee of the Red Cross (ICRC) said on Twitter that its team at a hospital in Saada received the bodies of 29 children, all under the age of 15.
It also received 48 wounded people, including 30 children.
Saada province, a Houthi stronghold, lies along the border with Saudi Arabia.
Colonel Turki al-Malki, a spokesman for the Saudi-led coalition, said the attack in Saada targeted the rebels who had fired a missile at the kingdom’s south, killing one person and wounding 11 others.
The coalition said the projectile, fired toward the southwestern Saudi city of Jizan, was intercepted and destroyed but its fragments caused Thursday's casualties.
The statement added that the missile was launched “deliberately to target residential and populated areas”.
Col al-Malki insisted Thursday’s attack carried out in Saada is a “legitimate military action” and is “in accordance with international humanitarian law and customs”.
He also accused the Houthis of recruiting children and using them as human shields.
Saudi Arabia backs Yemen’s internationally recognised government and has been at war with the Houthis since March 2015.
The rebels control much of northern Yemen, including the capital, Sanaa.
“Scores killed, even more injured, most under the age of ten,” the head of the ICRC in Yemen, Johannes Bruwer, said on his Twitter account, adding that the ICRC in Yemen is “sending additional supplies to hospitals to cope with the influx”.
Mohamed Abti from the Norwegian Refugee Council added that Thursday's attack was "not the first time civilians have been targeted by parties to the conflict", adding that both sides in the civil war must "respect international humanitarian law and make sure that civilians are protected".
Mr Abti added that because so many schools have been destroyed in the war, 2.2 million children are out of school, leaving them "vulnerable" to attacks.
Later on Thursday, air strikes hit the Yemeni capital, Sanaa, and sounds of the blasts reverberated across the city’s southern and western areas.
There was no immediate word on casualties.
Yemen’s three-year war has killed more than 10,000 people, badly damaged Yemen’s infrastructure, crippled its health system and led to one million people being infected with cholera.
The coalition faces widespread international criticism for air strikes in Yemen that kill civilians.
Impoverished Yemen, on the southern tip of the Arabian Peninsula, is in the grip of one of the world’s worst humanitarian crises, with more than 22.2 million people in need of assistance.