Saudi Arabia will investigate whether one of its airstrikes in Yemen was responsible for the killing of 40 children, state news agency SPA reported on Friday.

An airstrike hit a school bus at a market in Yemen's northern province of Saada on Thursday, killing at least 51 people, including the children. Yemen's Houthi rebels blamed the Saudi-led coalition for the attack, part of a fierce bombing campaign.

"The coalition has a firm commitment to conducting investigations into all incidents about which there are claims of mistakes, or violations of the international law," an unnamed coalition official was quoted as saying by SPA.

The Saudi military had earlier defended the strike as a "legitimate military action" and blamed the Houthis for using children as battlefield cover.

Calls for 'credible' probe

The United Nations has called for a "credible and transparent" investigation into the attack.

The German Foreign Ministry condemned the strike, saying that all such violations of human rights and international law should be completely and independently investigated.

Read more: Yemen's war explained in 4 key points

"The death of so many children and the grief and suffering of their families is deeply depressing," a spokesman said. "We call on the warring parties to adhere to international humanitarian law."

The UK, the US and the Houthis also backed calls for a probe.

Read more: In Yemen, 'nearly all children' at risk from war

Houthis to cooperate

Senior Yemeni rebel leader Mohammed Ali al-Houthi said on Twitter that the rebels would cooperate in an independent investigation.

The 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.

The Saudi-backed government and the Houthi rebels have been fighting for four years now, devastating Yemen's health care system killing at least 60,000 people since 2015.

aw/rc (AP, dpa, Reuters, AFP)