(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.

"The UN is offering a way forward through a dialogue on peace. We hope that all belligerents get to the peace table and start negotiating an end to this terrible war," she said.

Saudi denies targeting children

Saudi Arabia denies targeting civilians and rejected a UN report last year that blacklisted the country for deaths and injuries to children in the Yemen war.

Earlier Thursday, the Saudi-led coalition defended the airstrike as a "legitimate military operation," and a retaliation to a Houthi ballistic missile that targeted the kingdom's Jizan province on Wednesday night, according to the country's official news agency. One person was killed in that attack, Saudi state media reported.

Col. Turki al-Malki, a spokesman for the Saudi-led coalition, said the airstrike that hit the bus was aimed at a "legitimate target."

"No, this is not children in the bus," he said. "We do have high standard measures for targeting (sic)."

Malki said those responsible for firing ballistic missiles and targeting civilians would "get what they deserve."

The United States called on the Saudi-led coalition to launch an investigation into Thursday's strike.

US State Department spokeswoman Heather Nauert said Washington did not have the "full details of what happened on the ground," but said "we're concerned about these reports."

"We call on the Saudi-led coalition to conduct ... an investigation," Nauert said.

The Pentagon, which provides support for the Saudi-led war, called on US allies to mitigate "noncombatant casualties."

"Our noncombat support focuses on improving coalition processes and procedures, especially regarding compliance with the law of armed conflict and best practices for reducing the risk of civilian casualties," it said in a statement.

More than 10,000 civilians have died and 40,000 have been wounded in the war, which has left 15 million Yemenis without access to clean water.

The country could also be facing its third major cholera epidemic, especially around Hodeidah, the World Health Organization has warned.