The brutal attack hit the bus in the city of Saada, whilst it was visiting a market.
Most of those onboard the bus were reportedly children on a school trip.
According to Yemen’s Health Ministry, which is controlled by the Houthi faction in Yemen’s civil war, 50 were killed and 77 injured.
The International Committee for the Red Cross says a local hospital received 29 bodies, predominantly of children younger than 15.
They also report the hospital took in 40 injured, 30 of them children.
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Red Cross spokeswomen Mirella Hodeib told CNN: “The hospital is very busy. They’ve been receiving wounded and dead since the morning and it is nonstop.
“Putting children in harm’s way is horrific and deplorable and making them pay such a price is unacceptable.”
The UN released a statement condemning the attack.
The statement said: “The Secretary-General condemns the air strike today by the coalition forces in Saada, which hit a busy market area in Majz District, and impacted a bus carrying children from a summer camp.
“He call for an independent and prompt investigation into this incident.”
Video from Houthi run Al-Masirah TV appears to show several children who lost limbs in the attack.
The footage also shows several children’s bodies under a blown-up bus.
Colonel Turki al-Malki, a spokesman for the Saudi-led coalition, denied the attack had killed children.
He said: “No there were not children on the bus.”
He insisted the strike was aimed at a “legitimate target”.
Yemen has been in a state of civil war since 2015 with Shia Houthis rebels, who are accused of having links to Iran, fighting against the Hadi government.
In March 2015 Saudi Arabia and other Arab states intervened on against the Houthis, and are continuing to do so.