"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, quoted by the state-run Saudi Press Agency.

The announcement came a day after the raid in the northern Yemeni rebel stronghold of Saada that also left at least 48 others wounded, stirring calls for an investigation from the United Nations and Washington.

The senior official said the decision to investigate the strike followed information reported by "media and international organisations working in Yemen, regarding an operation by coalition forces in the province of Saada Thursday, and of a passenger bus which suffered collateral damage in this operation".

A hospital supported by the International Committee for the Red Cross received the bodies of 29 children after the strike, the organisation said.

Huthi health minister Taha al-Mutawakel on Friday put the death toll at 51 people, including 40 children, while warning the number of victims could rise further.

UN Secretary-General Antonio Guterres called for an "independent and prompt" probe into the incident, while the US State Department urged the coalition to undertake a "thorough and transparent" investigation.