A Saudi-led coalition battling in Yemen on Wednesday dismissed as inaccurate a report by United Nations investigators that highlighted possible war crimes by all sides in the conflict-torn country including deadly air strikes by the alliance.

"We affirm the inaccuracies in the report and its non-neutrality," the coalition said in a statement released by the official Saudi Press Agency.

"The report did not mention Iran's role in the continuation of the war... and its continued support for the Huthi" militias.

The coalition added that it would later provide a "comprehensive and detailed legal response" to the report.

In their first report released on Tuesday, a team of UN-mandated investigators said all parties in Yemen's bloody conflict have committed a "substantial number of violations of international humanitarian law".

Many of these violations may amount to "war crimes", the report said, pointing to widespread arbitrary detention, rape, torture and the recruitment of children as young as eight to take part in hostilities.

The report said coalition air strikes had caused "most of the documented civilian casualties", pointing to a large number of strikes on residential areas, markets, funerals, weddings and medical facilities.

It said there were "serious concerns about the targeting process applied by the coalition," pointing out that in many cases there were no apparent military targets in the vicinity of the attacks.

The report covers the period from September 2014 through June 2018, and does not address the latest series of deadly strikes that have killed dozens of children in rebel-held areas and sparked international outrage.

The coalition has not confirmed or denied it carried out two air raids last Thursday that the UN said killed at least 26 children and four women south of the flashpoint rebel-held city of Hodeida.

Those deaths came after a coalition attack on a bus in the northern rebel stronghold of Saada early this month killed 40 children, prompting UN Secretary-General Antonio Guterres to call for an independent investigation of attacks targeting civilians in the three-year war.

US support for coalition 'not unconditional'

The United States on Wednesday said the report was “very concerning”.

“We’ve seen that report from the Human Rights Council,” State Department spokeswoman Heather Nauert told reporters, saying that possible violations of international law “as outlined in that report are very concerning to the US government.”

“We believe that if such crimes have taken place that there is simply no justification,” she said, calling on parties to the conflict to “take necessary measures to prevent such violations.”

US Defense Secretary Jim Mattis on Tuesday strongly defended the US-backed Saudi-led coalition fighting Iran-backed Huthi rebels in Yemen.

But Mattis added that US support was “not unconditional,” the conditions being that the coalition does “everything humanly possible to avoid any innocent loss of life, and they support the UN-brokered peace process.”

Children ‘most vulnerable’

In addition to air strikes, children in Yemen are falling victim to other forms of violence.

"Children are the most vulnerable. Poverty, coupled with violence and insecurity render them defenceless and exposed to exploitation and abuse by parties to the conflict," Charles Garraway, a member of the expert group, told reporters.

The experts said they had received "substantial information" indicating that Yemen's government, the coalition-backed forces and the rebels had all conscripted children, mainly aged 11 and up, but some as young as eight.

The report also decried the devastating impact a blockade imposed by the Saudi coalition on Yemen's ports and the Sanaa airport had had on the civilian population.

Kamel Jendoubi, who heads the UN's so-called Group of Independent Eminent International and Regional Experts, urged an end to "disproportionate restrictions on the safe and expeditious entry into Yemen of humanitarian supplies and other goods indispensable to the civilian population."

Yemen's devastating conflict has left nearly 10,000 people dead since March 2015, when a Saudi-led coalition intervened to fight Huthi rebels closing in on the last bastion of President Abedrabbo Mansour Hadi's government.

The UN human rights office said Tuesday that some 6,660 civilians were among the dead, while more than 10,500 had been injured.

The UN has already described the situation in Yemen as the world's worst humanitarian crisis.

The report’s experts did not delve into the complex web of countries supporting different parties to Yemen’s conflict, including the US, Britain and Iran, but did call on all nations to stop selling arms that could be used in the war.

They also called on all sides to halt hostilities, and urged support for UN efforts to broker a peace deal.

UN-backed talks between Yemen's government and the Iran-aligned Huthi militias are to open in Geneva on September 6 -- a first step toward resuming peace negotiations that broke down two years ago.

"This crisis has reached its peak with no apparent sight of light at the end of the tunnel," Garraway said.