(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.

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