The pictures show bloodied children crawling on a road in Yemen. More lie motionless in the dirt and dust behind them.

In video from a local hospital, doctors receive dozens of bodies wrapped in white sheets. Those lucky enough to have survived the airstrike are treated for their injuries. Some of the small figures are still wearing their backpacks.

The images show the aftermath of a U.S.-backed, Saudi-led coalition airstrike in northern Yemen on Thursday. It left more than 50 people dead — mostly children traveling on a bus coming home from summer camp, according to the U.N.

News of the bombing reverberated around the world, and focused attention on the three-year war. It also raised the question: In the midst of the world's worst humanitarian crisis, what can be done to protect Yemen's most vulnerable?

The calls for action were almost immediate.

The U.N. said Friday that it believed the airstrike was the "single worst attack since 2015."

“How many more children will suffer or die before those who can act, do by putting a stop to this scourge?" asked Henrietta Fore, the executive director of UNICEF, the U.N.'s children's fund.

According to the agency, some 2,400 children have been killed and 3,600 maimed in the country since the conflict between the pro-Government forces and Houthi rebels escalated in 2015. Around 1.8 million Yemeni children are at risk of diarrhea diseases and 1.3 million are at risk of pneumonia.

The Saudi-led coalition — whose members get Western political backing and buy billions of dollars worth of arms from the United States, Britain and France — has waged a campaign against Yemen's Houthi rebels since early 2015. The coalition is fighting to counter the influence of arch-foe Iran, an ally of the Houthis.