Commenting on the coalition investigation, James Dorsey, a Middle East specialist at the S. Rajaratnam School of International Studies in Singapore, said that "unsupervised investigations of oneself are always problematic."

After a meeting last Friday, the UN Security Council called for a "credible" probe but it did not request an independent investigation -- as demanded by UN Secretary-General Antonio Guterres.

The coalition, which includes the United Arab Emirates, announced afterwards that it had ordered an investigation into the strike in Saada which it claimed initially had targeted Huthi rebel combatants.

The raid last Thursday hit a bus at a crowded market in northern Yemen, killing the 29 children and injuring around 50 other people, according to a toll from the International Committee for the Red Cross.

The United Nations has called for a "credible" investigation into an airstrike that killed at least 29 children in Yemen, but experts and aid groups doubt that a probe announced by the Saudi-led coalition bombing the country will yield results.

"Add to this that Saudi Arabia’s record in such investigations, its lack of transparency, and human rights record offers little reason to have confidence in an investigation that it controls," he added.

The coalition inte

rvened in Yemen in 2015 to try to restore the internationally recognised government of President Abedrabbo Mansour Hadi after Iran-backed Huthi rebels drove him out of the capital Sanaa.

The war has left around 10,000 people dead, the vast majority of them civilians, and caused the world's worst humanitarian crisis, according to the United Nations.

The coalition has been repeatedly blamed for bombing civilians, including during a strike on a wedding hall in the town of Mokha in September 2015, where 131 people died. The alliance denied responsibility.

In October 2016, a coalition air strike at a funeral in Sanaa killed 140 people.

The coalition, armed with Western-made aircraft and other weapons, has admitted a small number of mistakes, but accuses the rebels of using civilians as human shields.

"An airstrike on a bus full of schoolboys appears to be a flagrant violation of the rules of war, but in the absence of an independent, professional investigation we cannot say for sure," Sheila Carapico, a professor of international studies at the US-based University of Richmond, told AFP on Monday.

"Unfortunately, the Saudi military is likely to resist formal outside examination of the evidence, and their main weapons suppliers in the UK and the US seem reluctant as well," she added.

"The kingdom of Saudi Arabia, with no record or experience in this kind of investigation, almost always issues blanket denials."

"Très improbable"

Major Western countries have condemned the air strikes, but they remain political and military backers of Saudi Arabia, which is a regional ally and spends billions of dollars on arms from the United States, Britain and France.

During US-led air campaigns in Afghanistan, Iraq and Syria since 2001, Western forces occasionally admitted "collateral damage" when civilians were killed unintentionally.

But they too resisted independent investigations into the circumstances of major errors.

"The sad truth is the Saudis have bee

n given a chance to investigate themselves and the results are laughable," said Human Rights Watch's deputy UN director Akshaya Kumar last Friday.

The coalition initially called the strike on the school bus a "legitimate military action" in response to a rebel missile attack on Saudi Arabia's southern Jizan city a day earlier.

But as international media photographed dazed and bloodied children flooding into hospitals, it was forced to concede to an investigation.

The bombing raid, part of a military intervention that reflects Saudi Crown Prince Mohammed bin Salman's increasingly assertive foreign policy, follows the kingdom's diplomatic rupture with Canada last week.

Saudi Arabia expelled Canada's ambassador, recalled its own envoy and froze all new trade and investments after Ottawa publicly demanded the "immediate release" of rights campaigners jailed in the kingdom.

The Saudi reaction could impinge on the kingdom's efforts to attract badly needed foreign investment to fund its ambitious reform plan to pivot the economy away from oil, experts say.