UN Secretary General António Guterres has condemned a recent deadly airstrike by Saudi warplanes on a bus carrying Yemeni school children, calling for an independent investigation into the case.

"The secretary general condemns the airstrike today by the coalition forces in Sa’ada, which hit a busy market area in Majz district and impacted a bus carrying children from a summer camp," Farhan Haq, the UN chief's deputy spokesman, said in a statement on Thursday.

He added that Guterres has urged an “independent and prompt” probe into the attack that killed at least 50 civilians, mainly children, and wounded 77 others at a market in Yemen's northwestern province of Sa'ada.

The Saudi/UAE/US coalition just bombed a school bus. It’s an atrocity. And if Congress has any backbone, we would have taken action by now to stop US involvement in this unauthorized war.https://t.co/HqSrj9HEm2 — Rep. Barbara Lee (@RepBarbaraLee) August 9, 2018

According to the spokesman, Guterres has stressed that “all parties must take constant care to spare civilians and civilian objects in the conduct of military operations.”

The bus targeted by the coalition forces was carrying a group of young school children attending summer classes of the Holy Qur'an, Yemen's al-Masirah television network reported.

Johannes Bruwer, the head of the International Committee of the Red Cross (ICRC) delegation to Yemen, said in a tweet that most of those killed by the airstrike were children less than 10 years of age.

The Saudi-led coalition, in a defiant statement, has described the massacre as a “legitimate action” to target missile launchers used by Houthi Ansarullah fighters to target the southern Saudi city of Jizan.

Coalition spokesman Turki al-Malki even claimed that the strikes "conformed to international and humanitarian laws.”

By backing the Saudi coalition’s war in Yemen with weapons, aerial refueling, and targeting assistance, the United States is complicit in this atrocity. No one can seriously claim that our support for this war is actually making us safer. https://t.co/gP1HPpMYZ3 — Bernie Sanders (@SenSanders) August 9, 2018

The Al Saud regime along with some of its allies, particularly the UAE, has been waging a deadly war against impoverished Yemen since March 2015 in an attempt to reinstall former president Abd Rabbuh Mansur Hadi and crush the popular Ansarullah movement.