"The air strikes targeted a bus full of school students in the middle of Dahyan market in Saada province while they were going to their school", the official said.
A TV station run by the rebel Houthi movement reports that it was hit by a Saudi-led coalition air strike.
Footage from the Houthi media office showed a boy wearing a blue backpack with a UNICEF logo being carried into a hospital emergency room with blood pouring down his face and over his traditional Yemeni thawb, an ankle-length garment.
Coalition spokesman Turki al-Maliki told AFP that claims by aid organisations that children were inside the bus were "misleading", adding that the bus carried "Huthi combatants".
It was not immediately clear whether the bus was the target of airstrikes, but coalition spokesman Col.
The alleged attack left one Yemeni resident dead and 11 civilians wounded, said Maliki.
An airstrike in the north reportedly killed at least 50, more than half of them children.
In another video, which appears to show the immediate aftermath of the strike, several lifeless bodies of children lay under a blown-up bus.
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".
The Saudi-led coalition, in a defiant statement, has described the massacre as a "legitimate action" to target missile launchers used by Houthi Ansarullah fighters to target the southern Saudi city of Jizan.
Image It has been difficult to establish the number of people injured
He accused the rebels of using children as "tools and covers for their terrorist acts".
Houthi militants responsible for firing ballistic missiles and targeting civilians "will be held accountable", said the Saudi statement.
The Houthis seized the capital, 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.
"Today's attack in Saada was a legitimate military operation. and was carried out in accordance with worldwide humanitarian law", the coalition said in the Arabic-language statement carried by SPA. Just last week at least 55 people died in the city during a particularly bloody exchange of fire in residential areas and outside a hospital.
The coalition denied responsibility for those attacks.
Later, air strikes were reported in the rebel-held Yemeni capital, Sanaa.
"It is beyond cruel; innocent children's lives have been lost".
The Yemen war has killed more than 10,000 people, displaced more than two million and driven the country to the verge of starvation, according to the United Nations.
The United Nations has called for all parties involved in the conflict in Yemen to engage in peace talks.
China warns of tariffs on imported United States goods worth $60bn
The tariffs would be imposed at four different tax rates, China's Ministry of Commerce said in a statement . In negotiations with China, Trump's major request was a massive reduction in the bilateral trade deficit .