UN Secretary-General António Guterres on Thursday condemned a Saudi-led coalition air strike in Yemen that killed dozens of people, calling for "an independent and prompt investigation," a spokesman said.
"The Secretary-General emphasizes that all parties must take constant care to spare civilians and civilian objects in the conduct of military operations," UN spokesman Farhan Haq said in a statement quoted by Reuters.
The attack on a bus at a market in rebel-held northern Yemen killed at least 29 children, the Red Cross said, according to the AFP news agency.
The coalition said it had carried out a "legitimate military action", targeting a bus in response to a deadly missile attack on Saudi Arabia on Wednesday by Houthi rebels.
Coalition spokesman Turki al-Maliki told AFP that claims by aid organizations that children were inside the bus were "misleading", adding that the bus carried "Houthi combatants".
The International Committee of the Red Cross said the strike hit a bus filled with children at the Dahyan market in the Houthi stronghold of Saada.
"A hospital supported by our team in Yemen received the bodies of 29 children under the age of 15 and 48 wounded, including 30 children," the ICRC said on Twitter.
A spokesman for the Red Cross in Sanaa told AFP the toll was not final as casualties from the attack were taken to several hospitals.
State Department spokeswoman Heather Nauert said the U.S. is "calling the Saudi-led coalition to conduct a thorough and transparent investigation into the incident."
"We call on the parties to take appropriate measures to protect civilians," she added.
The Saudi-led Arab military coalition has been fighting the Iranian-backed Houthi rebels over the last few years.
It has long been believed that Iran is planning to use the Houthis to take over Yemen and seize the key strategic port of Aden, which controls the entrance to the Red Sea and ultimately to the Israeli resort city of Eilat.
Saudi forces have previously shot down Houthi missiles aimed at its territory with Patriot surface-to-air missiles purchased from the United States.