International relief agencies expressed outrage and disgust at a Saudi-led coalition airstrike on Houthi rebels that killed at least 43 people Thursday in Yemen, many of them children.
More than 60 people were wounded.
One missile struck a bus taking children back from a summer school picnic.
Television pictures from a hospital showed blood-covered youngsters who seemed too stunned to even cry. Others writhed on the floor, waiting for help.
"Grotesque, shameful, indignant," Norwegian Refugee Council head Jan Egeland tweeted. "Blatant disregard for rules of war when a bus carrying innocent schoolchildren is fair game for attack."
Save the Children said it "condemns this horrific attack and is calling for a full, immediate, and independent investigation into this and other recent attacks on civilians and civilian infrastructure."
Doctors Without Borders decried that "civilians continue to pay the highest price" in Yemen, while the regional director of the U.N. Children's Fund asked, "Does the world really need more innocent children's lives to stop the cruel war on children in Yemen?"
The United States backs the Saudi-led coalition targeting Iranian-supported Houthi rebels in Yemen.
"We trust when they say that they will investigate. We closely coordinate with them," State Department spokeswoman Heather Nauert said.
WATCH: Saudi-led Coalition Airstrike in Yemen Kills Dozens, Many of Them Children
Response to Houthi strike
Thursday's airstrike was carried out against a rebel-held area in Saada province, near the Saudi border. Missiles struck a market.
It came in response to a Houthi missile strike on Saudi territory Wednesday. Saudi defenses intercepted the missile, but fragments fell to the ground, killing one and wounding 11.
The coalition called Thursday's airstrike against the Houthis "a legitimate military operation ... carried out in accordance with international humanitarian law."
The Saudis have accused the Houthis of using children as human shields, a charge the rebels deny.
Iranian-backed Houthi rebels seized the Yemeni capital of Sanaa in 2014, sending the Western-supported government into exile in Saudi Arabia.
The Saudi-led coalition began its air and ground campaign to drive out the Houthis more than three years ago. Its airstikes have obliterated civilian neighborhoods, including schools and hospitals, and compounded the misery of what is one of the world's worst humanitarian disasters.
Many Yemenis are near starvation. There are also severe shortages of fresh water and medicine. A cholera epidemic has killed thousands over the last two years.