The Saudi military coalition – which receives logistical support, weapons and political backing from the US and UK – has been accused of killing hundreds of children in Yemen, according to a confidential UN report.

The report, which has yet to be made public and could still be changed, says that 51 per cent of all child deaths and injuries in Yemen last year were the result of the Saudi-led military operation. It says the deaths were “unacceptably high”. Saudi Arabia has insisted it is operating within international law.

“Attacks carried out by air caused over half of all child casualties, with at least 349 killed and 333 injured,” said the report, which was obtained by Reuters.

“The United Nations was informed of measures taken by the coalition in 2016 to reduce the impact of conflict on children. However, despite these measures, grave violations against children continued at unacceptably high levels in 2016.”

Saudi Arabia has always insisted that its operations follow international guidelines. Its UN mission said in a statement there was “no justification whatsoever” for including the coalition’s name on the blacklist.

“We trust that the United Nations will make the appropriate decision on this matter, and the positive exchange of information” on the coalition’s activities, the statement said. It declined to comment on the findings in the draft report for 2016.

Saudi Arabia is leading a nine-nation coalition in a bombing campaign that started in March 2015 to defeat Iran-allied Houthi rebels. The US and UK have offered logistical and political support.

Britain has also continued to sell weapons to Saudi Arabia despite mounting worries over civilian deaths, believed to total around 3,000. Last month, a British court ruled that such sales could continue despite humanitarian concerns and rejected an appeal by the Campaign Against Arms Trade to stop them.

The role of the UK and US in supporting Riyadh has come under mounting scrutiny as concern about civilian deaths has grown.

Yemen crisis: More than one million children suffering from malnutrition

Last year, the UK Government considered suspending arms sales after a strike in October 2016 killed 140 civilians attending a funeral. In the end, Britain, which has sold around $3.8bn of weapons to Saudi Arabia over the past two years and is its biggest supplies of arms, decided to continue with the sales.

In May, US President Donald Trump visited Saudi Arabia where he delivered a major speech before Muslim leaders and announced multiple agreements between American and Saudi companies, including between the Saudi oil giant Aramco.

“That was a tremendous day. Tremendous investments in the United States,” he said. “Hundreds of billions of dollars of investments into the United States and jobs, jobs, jobs.”

Andrew Smith of Campaign Against Arms Trade said of the UN report: “The humanitarian crisis in Yemen is among the worst in the world, and the bombardment is making it even worse. This couldn't have happened without the complicit support of governments like the UK, which have armed and supported Saudi forces every step of the way.

“For decades now, the Saudi regime has had one of the worst human rights records in the world, and that brutality has been on full display in its destruction of Yemen.”

Reports suggest it will be up to UN Secretary-General Antonio Guterres to decide whether the report, which blamed the Saudi coalition for more than 680 child casualties and three-quarters of the attacks on schools and hospitals, is made public. It was written by the UN special envoy for children and armed conflict, Virginia Gamba.

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Reuters said it will also be up to Mr Guterres whether to return the Saudi-led coalition to a child rights blacklist annexed to the report. The coalition was briefly added last year and then removed by then-UN chief Ban Ki-moon pending a review.

The Saudi-led coalition had been named on the blacklist last year after the UN report blamed it for 60 per cent of child deaths and injuries in Yemen in 2015 and half the attacks on schools and hospitals.

A spokesman for the UK Foreign Office said: “We call on all parties to the conflict in Yemen to avoid any civilian casualties, particularly that of children, and to permit humanitarian access.”

Noel Clay, a spokesman for the US State Department, said: "We wouldn’t comment on a draft report and would refer you to the UN for anything further."