Saudi-led coalition airstrike kills dozens of children on bus in Yemen The children killed were reportedly under the age of 15.

LONDON -- An airstrike by the Saudi-led coalition in Yemen on Thursday morning killed at least 50 people including dozens of children traveling on a bus in the country’s Saada Province, local health officials said.

The International Committee of the Red Cross said its medical teams received the bodies of 29 children, all under the age of 15. They also received 48 injured people, including 30 children, the ICRC said.

Yemen’s rebel-run Al Masirah TV aired footage of injured children weeping as blood streamed down their faces. Some of the children carried blue UNICEF backpacks, spotted with blood.

Col. Turki al-Malki, a spokesman for the Saudi-led coalition in Yemen, which has the backing of the U.S. government, said the coalition had launched an operation in Saada in response to Houthi fighters firing a missile on the Saudi city of Jizan on Wednesday evening.

Houthi spokesman Mohammed Abdul-Salam said the Saudi-led coalition had showed no regard for civilian lives by targeting a school bus in a crowded public space.

State Department spokeswoman Heather Nauert on Thursday called on the Saudi-led coalition to "conduct a thorough and transparent investigation into the incident," and for all parties to protect civilians in accordance with international law.

Lt. Cmdr. Rebecca Rebarich, a Pentagon spokeswoman, said in a statement that U.S. Central Command was not involved in the airstrike in Saada.

"U.S. military support to our partners mitigates noncombatant casualties, by improving coalition processes and procedures, especially regarding compliance with the law of armed conflict and best practices for reducing the risk of civilian casualties," she said.

Save the Children’s staff on the ground in Saada said the children were on their way back to school from a picnic when the attack happened.

Sylvia Ghaly, director of advocacy in Yemen for Save the Children, said the children were between the ages of 6 and 15.

“From where we sit as humanitarian actors, we don’t see the military targets, we see civilians being targeted and children being killed, and at the same time we don’t see anyone being held accountable for the attacks,” she told ABC News. “It’s not good enough to say that this was a mistake or that it was collateral damage. At the end of the day, that child has a name and that child is the son or daughter of someone who will grieve for a long time.”

She added that Save the Children is calling for an immediate, independent investigation into the attack. Ghaly said that when she entered the province of Saada, a Houthi stronghold, many buildings had basically been reduced to rubble.

Meritxell Relano, UNICEF’s representative in Yemen, said children of Yemen face a bleak future.

“The future of the children of Yemen is dark,” she told ABC News. “Right now, I don’t see the light at the end of the tunnel -- no peace, no agreement, no cessation of hostilities. An entire generation of children will be lost and how will this country be reconstructed in the future?”

Yemen is one of the world’s poorest countries and the war has made conditions much worse: More than 22 million people are in need of humanitarian assistance and half of the country’s health facilities are out of service.

The Saudi-led coalition has been fighting Houthis in the country since March 2015 after the Houthis took over the capital of Sanaa and forced interim president Abdrabbuh Mansour Hadi and his government to flee the country. An Arab Sunni coalition led by Saudi Arabia then launched a war to restore Hadi’s government to power -- a military campaign that is supported by the U.S. The coalition has been blamed by the United Nations for most of the civilian deaths in Yemen.

ABC News' Elizabeth McLaughlin contributed reporting from the Pentagon, and ABC News' Sarah Kolinovsky contributed reporting from Washington.