The UN Security Council called for a "credible" investigation Friday after at least 29 children were killed in an air strike by the Saudi-led coalition in northern Yemen.
The appeal followed the coalition's announcement that it had ordered a probe of Thursday's raid in the northern rebel stronghold of Saada.
Britain's Ambassador Karen Pierce, whose country holds the rotating Security Council presidency, said after a closed-door meeting on Yemen that "if any investigation that is held is not credible, the council will obviously want to review that" and decide "if more is necessary."
The raid hit the bus at Dahyan market in Saada, injuring at least 48 others, including 30 children, according to the International Committee for the Red Cross.
The bus was turned into a mass of twisted metal, and remains of the victims and personal items were still scattered across the ground on Friday.
The coalition, which has been fighting Yemen's rebels since 2015, claimed the bus was carrying "Huthi combatants."
It initially said the coalition had carried out a "legitimate military action," targeting a bus in response to a deadly missile attack on Saudi Arabia by Huthi rebels on Wednesday.
At a press conference in Dahyan, 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.
- Children 'excited' for trip -
At the time of the attack, the children were on a bus heading back to school "from a picnic," the Save the Children charity said, quoting its staff.
"The mothers told me that their children did not sleep for two days because they were too excited to take part in this trip," Yahya Hussein, one of the children's teachers, told AFP.
The Huthis' Islamic affairs ministry said the children were from a Koranic school.
As international outrage over the attack mounted, the coalition said it would open an investigation.
US Ambassador Nikki Haley said the images of the strike that killed children were "appalling" and called on the coalition to quickly complete its investigation, release the findings and take accountability measures.
The Security Council met behind closed doors to discuss the attack, at the request of five countries that are non-permanent council members.
"We have seen the images of children who died," Dutch Deputy Ambassador Lise Gregoire-van Haaren told reporters. "What is essential at this moment in time is to have a credible and independent investigation."
But the council did not request an independent investigation -- as demanded by UN Secretary-General Antonio Guterres, who condemned the attack.
Human Rights Watch criticised the council's failure to demand an impartial investigation.
"The sad truth is the Saudis have been given a chance to investigate themselves and the results are laughable," said HRW's deputy UN director Akshaya Kumar.
Out of 75 cases of civilian deaths investigated by the coalition, only two have resulted in an admission of fault, she said.
- Rebel missile attacks -
The coalition, which also includes the United Arab Emirates, intervened in Yemen to try to restore the government after the rebels drove it out of the capital Sanaa.
Coalition spokesman Turki al-Maliki said claims by aid organisations that children were inside the bus were "misleading," and that "the elements inside the bus were Huthi combatants."
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.
The coalition said it intercepted and destroyed two more ballistic missiles fired by the Huthis from Saada towards Jizan. No damage or injuries were reported.
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.
UN envoy to Yemen Martin Griffiths has invited the warring sides to talks on September 6 in Geneva.