Thursday's strike on the Dahyan market in the Huthi rebel stronghold of Saada injured at least 48 others, including 30 children, according to the International Committee for the Red Cross.

An AFP photographer at the shocking scene said the bus carrying the children had been turned into a mass of twisted metal, and that the remains of victims and their personal items were still scattered across the ground.

"There are remains everywhere, we are still trying to confirm identies," Yahya Shayem, a health official in Saada, told AFP.

He could not confirm when funerals for the victims would take place.

The coalition, which has been fighting Yemen's rebels since 2015, acknowledged responsibility for the strike, but claimed the bus was carrying "Huthi combatants".

It said the coalition had carried out a "legitimate military action", targeting a bus in response to a deadly missile attack on Saudi Arabia on Wednesday by Huthi rebels.

At a press conference in Dahyan on Friday, Huthi health minister Taha al-Mutawakel put the death toll from the "horrible crime" at 51 people, including 40 children.

"This toll is not final... a lot of people are missing and the remains are still scattered around the crime scene and nearby," he said.

The Red Cross could not immediately confirm the new figures.

Thursday's strike sparked calls from both United Nations chief and the US State Department for the strike to be investigated.

UN Secretary-General Antonio Guterres urged an "independent and prompt" probe, while State Department spokeswoman Heather Nauert said Washington was "calling the Saudi led coalition to conduct a thorough and transparent investigation into the incident".

"We call on the parties to take appropriate measures to protect civilians," Nauert said.

- 'Massacre of children' -

In comments Friday on Twitter, the rebels' revolutionary council head, Mohammed Ali al-Huthi, welcomed the UN call for an investigation and said his side was "willing to cooperate".

Ahmed al-Mansouri, the director of Saada's Al-Jumhuri hospital, condemned what he called the "massacre of children".

The coalition, which also includes the United Arab Emirates, intervened in 2015 to try to restore the internationally recognised government after the rebels drove it out of the capital Sanaa.

Coalition spokesman Turki al-Maliki told AFP that claims by aid organisations that children were inside the bus were "misleading", and that "the elements inside the bus were Huthi combatants".

He said the rebels have "ballistic missiles threatening our national security, Saudi cities and villages on the border. The attack today was a follow-up process to neutralise that threat."

Earlier, Maliki accused the Huthis of "recruiting child soldiers, throwing them in battlefields and using them as tools".

Saudi Arabia shot down a missile fired by the Huthis on Wednesday, with debris killing a Yemeni man and wounding 11 others, the coalition said.

The missile was fired from the rebel-held Yemeni province of Amran towards the Saudi city of Jizan, the coalition said.

- Huthis ramp up missile attacks -

The Huthis have in recent months ramped up missile attacks against Saudi Arabia, which Riyadh usually says it intercepts.

Wednesday's attack brings the tally to 167 rebel missiles launched since 2015, according to the coalition.

On August 2, attacks on a hospital and a fish market in the strategic rebel-held port city of Hodeida killed at least 55 civilians and wounded 170, according to the ICRC.

The coalition denied responsibility for those attacks.

Aid agency CARE International noted that Thursday's strike came a week after the Hodeida bombardment.

"This latest air strike, only a week after the attacks on Hodeida city, demonstrates a continued disregard for human life and suffering," said Johan Mooij, the agency's country director in Yemen.

"It is beyond cruel; innocent children's lives have been lost."

The war in impoverished Yemen has left nearly 10,000 people dead and unleashed what the UN describes as the world's worst humanitarian crisis.

The United Nations envoy to Yemen, Martin Griffiths, last week told the Security Council "a political solution" to Yemen's war was "available" and that the warring sides would be invited to talks on September 6 in Geneva.

UN-brokered negotiations on Yemen broke down in 2016 amid demands for a rebel withdrawal from key cities and power-sharing with the Saudi-backed government.