CNN  — 

UN Secretary-General António Guterres has called for an independent investigation into a Saudi-led coalition air strike in Yemen that killed dozens of children.

The airstrike on Thursday hit a bus carrying children from a summer camp in a busy market area in the northern Majz District, UN spokesman Farhan Haq said in a statement.

Condemning the attack, Guterres called for “an independent and prompt investigation” into the incident, Haq said.

In the statement, Guterres added that all parties must “respect their obligations under international humanitarian law, in particular the fundamental rules of distinction, proportionality and precautions in attack.”

According to the area’s Houthi-controlled Health Ministry, 50 people were killed and 77 injured in the strike. The International Committee for the Red Cross said a hospital it supports in northern Saada province had received 29 bodies of “mainly children” younger than 15, and 40 injured, including 30 children.

Footage from Houthi-run Al-Masirah TV appears to show a boy, carrying a UNICEF backpack, being treated for injuries.

Houthi media broadcast graphic footage appearing to show the bodies of children. CNN has not independently verified these images.

A video from Houthi-run Al-Masirah TV shows several boys who appear to have lost their limbs. Two or more wounded children are seen sharing a single hospital bed, and one child – soaked in blood – screams as he is being treated at a health center.

In another video, which appears to show the immediate aftermath of the strike, several children’s bodies lie under a blown-up bus. Some boys are seen regaining consciousness, their faces bloodied and limbs charred.

The boy is treated in a hospital after the strike.

One boy, his face blackened by dust, is seen trying to hold his legs up, apparently unable to move. “My leg won’t get up,” he says.

The attack came a week after a Saudi-led airstrike hit a busy fish market and the entrance to the country’s largest hospital, Al-Thawra, in the port city of Hodeidah, killing 55 civilians and wounding 170 others.

Since March 2015, Saudi Arabia has led a coalition of Gulf states against Houthi rebels in Northern Yemen, after the Iran-backed rebels drove out the US-backed and pro-Saudi government.

The war in Yemen is now the world’s worst humanitarian crisis, with more than 22 million people – three-quarters of the population – in desperate need of aid and protection, the UN says.

After Thursday’s strike, Guterres renewed his call for a negotiated political settlement ahead of consultations scheduled in Geneva in September.

Lise Grande, United Nations humanitarian coordinator in Yemen, urged all parties to come to the table.