The Saudi-led coalition battling Yemen's Houthi rebels announced on Friday an investigation into an airstrike on a bus that killed at least 29 children, as pressure mounted on the U.S.-backed military coalition over the deadly strike. The announcement came after the Shiite Muslim rebels backed international calls for a probe into the airstrike in the country's north which killed dozens the previous day.

The attack has drawn wide international criticism. Both the United Nations and the U.S. State Department called on the coalition, led by U.S. ally Saudi Arabia, to carry out an investigation.

"The leadership of the coalition has ordered the immediate opening of an investigation to assess the events, clarify their circumstances and announce the results as soon as possible," a senior coalition official said, according to the French news agency AFP.

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Earlier, senior Yemeni rebel leader Mohammed Ali al-Houthi said on Twitter that the Houthi rebels welcomed the call and are willing to cooperate in an investigation of the strike that hit a bus carrying civilians, many of them school children, in a busy market in Dahyan district.

Yemenis dig graves for children who where killed when their bus was hit during a Saudi-led coalition airstrike that targeted the Dahyan market the previous day in the Houthi rebel stronghold province of Saada, Aug. 10, 2018. Getty

In a statement after Thursday's airstrike, U.N. chief António Guterres urged Yemen's warring parties to take "constant care to spare civilians" during military operations and also called for an "independent and prompt investigation."

The United Nations said an exact death toll had yet to be confirmed but initial reports point to more than 60 casualties, with dozens severely wounded. The rebel-run Al Masirah TV reported at least 51 killed and 79 wounded in the airstrike, citing the Yemeni Health Ministry in the capital, Sanaa, which is under rebel control.

It also said three children have gone missing since the airstrike.

The International Committee of the Red Cross said its team received the bodies of 29 children, all under 15 years old, and treated 48 wounded, 30 of them children.

Following the strike, Al Masirah broadcast horrific images of lifeless bodies of children, covered in blood, and others who appeared severely wounded, lying on hospital stretchers crying and screaming in pain. The authenticity of the footage could not be independently verified.

On Friday, Ahmed al-Hamoud, who was traveling from Saada to Sanaa, said a somber mood prevailed over the province and that coalition planes could be seen flying over it from time to time.

The U.S. State Department called on the Saudi-led coalition "to conduct a thorough and transparent investigation into the incident," spokeswoman Heather Nauert said. "We take all credible accounts of civilian casualties very seriously."

Also, the U.N. special envoy for Yemen, Martin Griffiths, who has been pushing for peace efforts in the war-torn country, said he was "deeply shocked by the appalling tragedy that claimed so many innocent lives."

Still, he called on the warring parties to "engage constructively" in the first round of consultations scheduled for Sept. 6 in Geneva. The United Nations children's agency called the attack in Saada "unconscionable" and a "low point in the country's brutal war."

What is Saudi Arabia's role in the Yemen war?

The Saudi-led coalition, which has been at war with the Houthis for more than three years, said the attack on Saada was in response to a missile fired by the rebels into the kingdom's south a day earlier. The coalition said it had intercepted and destroyed the missile but its fragments killed one person and wounded 11 others in Saudi's southwestern border region of Jizan.

Coalition spokesman Col. Turki al-Malki defended the attack on Saada on Thursday as a "legitimate military action" and blamed the Houthis for recruiting children and using them in the battlefields to cover for their actions.

Impoverished Yemen has been embroiled in the war pitting the Saudi-led coalition against the Iran-aligned Houthis since March 2015. Iran, as the world's most powerful Shiite Muslim nation, and Sunni-led Saudi Arabia have been bitter foes for decades.

Civilians have been enmeshed over the years in the conflict which has killed over 10,000, crippled the country's health system and pushed it to the brink of famine.

Yemen has become the world's worst humanitarian crisis, with more than 22.2 million people in need of assistance.