The State Department has "seen the news reports" about a Saudi-led coalition airstrike that killed at least 50 people and wounded more than 70 - most of whom were children - in northern Saada, Yemen, but "can't confirm all the details because we are not there on the ground", Nauert told reports during a press briefing on Thursday.
As many as 35 were also wounded in the attack, which took place in the Dahyan market in Saada province, a stronghold of the rebels known as Houthis, the elders said.
The International Committee of the Red Cross said on Twitter that an ICRC supported hospital received dozens of dead and wounded following the attack that hit a bus with children.
"We call on the parties to take appropriate measures to protect civilians", Nauert said.
The Houthis' al-Masirah TV reported that 39 people had been killed and 51 wounded, saying they were "mostly children".
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A military coalition led by Saudi Arabia has been carrying out airstrikes since 2015 against Houthi positions - causing thousands of civilian deaths in the process, according to war monitors - in an attempt to restore the internationally-recognized government of President Abdrabbuh Mansur Hadi.
The missile was sacked from Amran province in northern Yemen on Wednesday night, the coalition said in a statement published by the Saudi Press Agency.
Yemeni tribal leaders told the AP the Saudi-led US -backed coalition fighting Shiite Houthi rebels was behind the airstrike.
Upon interception, Saudis say the missile exploded into fragments that killed one Yemeni civilian and injured 11 other people.
"Our support to the coalition consists of aerial refueling and intelligence support to assist our partners in securing their borders from cross-border attacks from the Houthis".
"Scores killed, even more injured, most under the age of 10", Johannes Bruwer, head of delegation for the ICRC in Yemen, said in a Twitter post.
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He accused the rebels of using children as "tools and covers for their terrorist acts".
Most of those onboard the bus were reportedly children on a school trip.
@AdamjohnsonNYC I just saw MSNBC do a short segment on how United States back Saudi Arabia dropped a bomb on a school bus in Yemen.
The war in impoverished Yemen has left almost 10,000 people dead and unleashed what the United Nations describes as the world's worst humanitarian crisis.
The Iran-aligned Houthis regularly fire into Saudi Arabia and have targeted its capital, Riyadh, with ballistic missiles.
There was a horrifying attack Thursday in Yemen - a Middle Eastern country torn apart by a 3-year-old civil war.
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