(CNN) - For one Yemeni father, any hope that his son was alive was crushed when he spotted a body in the back of a pickup, the corpse partially covered by a blue tarpaulin.
"Is this Yousif? Is this Yousif?" he cries as he grabs at the side of the vehicle and begins to shake it: "Oh my God!"
Pain is etched across his face as the reality sinks in.
"Oh Yousif, oh Yousif," he wails, his voice rising to a scream.
"What is the fault of these small children?" he asks of no one in particular.
In a second video, another father describes his desperate search for his son among the dead and injured.
"I didn't see him," he says. "I looked among the bodies and the injured. I didn't see him."
His hand trembling while on the phone, he tries to find out what clothes his son was wearing the day before -- so he can pass the information on to others helping in the hunt for his missing boy.
The videos capture snapshots of the horrific scenes that have played out in northern Yemen after a Saudi-led coalition airstrike hit a school bus Thursday. Eyewitnesses have verified what is depicted in the videos, which came from media controlled by the Houthi rebels.
The strike was the worst attack on children since Yemen's brutal war escalated in 2015, UNICEF said Friday.
Saudi Arabia has led a coalition of Gulf states against Houthi rebels in northern Yemen after the Iran-backed rebels drove out the US-backed and pro-Saudi government.
Houthi media broadcast graphic footage of the immediate aftermath of the strike. In one video, boys appear to have lost their limbs. In another, children's bodies lie under a blown-up bus. Some boys appear to regain consciousness, their faces bloodied and limbs charred.
Houthi Health Minister Taha al-Mutawakil said during a news conference Friday that the death toll from the airstrike was at 51, including 40 children. Seventy-nine people were wounded, 56 of them children, al-Mutawakil said.
In a statement that was aired on Houthi-controledl al-Masirah TV, al-Mutawakil said that the ages of the dead and wounded children are between 6 and 11 years old, and that some people remain unaccounted for.
The International Committee for the Red Cross said a hospital it supports in northern Saada governate had received 29 bodies of "mainly children" younger than 15, and 40 injured, including 30 children.
Three children are still missing, the Saada health office said Friday.
The Saudi-led coalition said Friday it has opened an investigation regarding the Yemen attack, according to the state-run Saudi Press Agency.
No joint funeral for security reasons
A joint funeral for the children was ruled out Friday, said Hasan AlHomran, an office manager to Houthi leader Abdul-Malik al-Houthi, citing security concerns.
Saudi Arabia denies targeting civilians and defended the incident on Thursday as a "legitimate military operation" and a retaliatory response to a Houthi ballistic missile that targeted the kingdom's Jizan province the day before.
Coalition spokesman Col. Turki al-Malki told CNN the airstrike that hit the bus was aimed at a "legitimate target."
"No, this is not children in the bus," he said. "We do have high standard measures for targeting."
Condemning the attack, UN Secretary-General António Guterres called for "an independent and prompt investigation" into the strike.
In a statement, Guterres said all parties must "respect their obligations under international humanitarian law, in particular the fundamental rules of distinction, proportionality and precautions in attack."
Meanwhile, Lise Grande, the UN humanitarian coordinator in Yemen, urged all parties to come to the table.
"The UN is offering a way forward through a dialogue on peace. We hope that all belligerents get to the peace table and start negotiating an end to this terrible war. The humanitarian crisis in Yemen is the worst in the world," she said.
More than 10,000 civilians have died and 40,000 have been wounded in the war, which reportedly has left 15 million Yemenis without access to clean water.
Also Friday, Saudi Arabi's air force intercepted two ballistic missiles fired by the Houthis over the kingdom's Jizan province, according to the Saudi Press Agency.
The Houthi-controlled defense ministry confirmed it had fired several missiles while aiming for "several vital Saudi military targets in Jizan."
The war in Yemen is now the world's worst humanitarian crisis, with more than 22 million people -- three-quarters of the population -- in desperate need of aid and protection, according to the United Nations.
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