The attack has been blamed on the Saudi-led coalition
At least 29 children are said to among those killed in an airstrike on a bus in Yemen.
The International Committee of the Red Cross said one of the hospitals it supports in the city of Sa'dah in the north west of the country "received dozens of dead and wounded" from the attack.
It later put the number of dead at 29 children, with dozens more injured:
New figures from the ICRC-supported hospital in Sa'ada, #Yemen. Our medical team has received:— ICRC (@ICRC) August 9, 2018
- the bodies of 29 children, all under 15 years old.
- 48 injured people, among them 30 children.
In the Yemeni capital, the rebel-run Al Masirah TV said the airstrike on a market had killed 39 people and wounded 51 - mainly children.
The attack has been blamed on the Saudi-led coalition waging war against Yemen's Houthi rebels.
The ICRC warned that "Under international humanitarian law, civilians must be protected during conflict."
Following an attack this morning on a bus driving children in Dahyan Market, northern Sa’ada, @ICRC_yemen- supported hospital has received dozens of dead and wounded. Under international humanitarian law, civilians must be protected during conflict. pic.twitter.com/x39NVB8G4p— ICRC Yemen (@ICRC_ye) August 9, 2018
The Save the Children charity, meanwhile, called for an 'immediate and independent investigation' into the incident.
In a statement, the group said: "Our staff are telling us that the pupils were on their way back to school from a picnic when the driver stopped to get a drink.
"That’s when the attack happened, while the bus was stationary. We are unable to verify the details at this moment."
Save the Children's Director of Advocacy in Yemen, Sylvia Ghaly, added that it's "the people of Yemen, not the warring parties, who paying the ultimate price" for the protracted conflict in the country.
Fighting between Iranian-backed Houthi rebels and the ousted Yemeni government – supported by a Saudi-led coalition of Arab states – has been ongoing in the region for three years.
The US has been supporting the Saudi coalition with arms sales, intelligence and military support throughout the war.
The UK and Canada have also been accused of backing the war through their ongoing arms sales to the Saudi's.
The war has left some eight million people facing starvation and ignited the worst humanitarian crisis in the world.
Additional reporting by Stephen McNeice