"We are calling the Saudi led coalition to conduct a thorough and transparent investigation into the incident," she said.
Nauert added the US takes credible accounts of civilian casualties "very seriously." "We call on the parties to take appropriate measures to protect civilians," she said.
A hospital in Saada province supported by the International Committee of the Red Cross "received the bodies of 29 children under the age of 15, and 48 wounded, including 30 children," the organization announced on Twitter.
The coalition acknowledged carrying out a "legitimate military action," but spokesman Turki al-Maliki told AFP claims that children were inside the bus were "misleading," adding that the bus carried "Huthi combatants."
Saada is a stronghold of Huthi rebels, whom the Saudi-led coalition are fighting in support of President Abedrabbo Mansour Hadi's forces.
The war in Yemen has left more than 10,000 dead since 2015, sparking what the UN says is the world's worst humanitarian crisis.
The US has lent its Saudi allies support in the form of arms, intelligence and aerial refueling.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)