The International Committee of the Red Cross said it was sending supplies to ICRC-supported hospitals to support an influx of casualties.
"We are unable to verify the details at this moment", Save the Children wrote on August 9.
Hours later, well after the blast's violent rumble gave way to sirens and the victim's screams, a clearer sense of the human cost has emerged: At least 29 children under the age of 15, some as young as 6 years old, were killed in the attack, according to the International Committee of the Red Cross.
The hospital also received 48 wounded people, among them 30 children.
The bus was travelling through Dahyan market in Saada province at the time.
Col. Al-Maliki affirmed that the leaders and militants responsible for launching ballistic missiles and targeting civilians would be held accountable under the Coalition's efforts to prevent terrorist elements from compromising regional and worldwide security.
One person was killed in that attack, Saudi state media reported. But the coalition denied carrying out any attacks in the city, saying it follows a "strict and transparent approach based on the rules worldwide law".
A child lies in hospital after the school bus he was on was allegedly attacked by the Saudi-led coalition.
The UN children's agency UNICEF said it was "very concerned with the initial reports of children being killed".
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Doctors Without Borders (MSF) said "civilians continue to pay the highest price after three years of war in Yemen, thousands of them have been killed, injured or maimed".
Houthi media broadcast gruesome footage appearing to show the dead bodies of children.
The Iranian backed Houthi rebels seized the capital of Sanaa in 2014 and overthrew the internationally recognised government led by President Abdrabu Mansur Hadi during a transition of power from former president Ali Abdullah Saleh.
The statement, carried by the official Saudi Press Agency, also said the missile was launched "deliberately to target residential and populated areas".
CBS News was witness to Yemen's war first hand in February as Saudi-backed soldiers were seen fighting against Houthi rebels for control of the country's barren hills.
The coalition said Wednesday's projectile, fired toward the southwestern Saudi city of Jizan, was intercepted and destroyed but its fragments caused the casualties.
"The attack carried out today by the coalition in Saada was against those people responsible for the ballistic missile attack last night ... the allegation [that civilians were targeted] is coming by the Houthis, and it's still an allegation", Col Al Maliki said.
The coalition acknowledged carrying out a "legitimate military action", but spokesman Turki al-Maliki told AFP claims that children were inside the bus were "misleading", adding that the bus carried "Huthi combatants". There was no immediate comment from the Houthis in response to the coalition allegations.