Weapons provided by Western powers and Iran to the warring sides in Yemen are fuelling the six-year-old conflict, marked by deadly Saudi-led coalition air strikes and Houthi shelling, UN investigators said on Wednesday.
Coalition air strikes in the past year may amount to war crimes, while the Iran-aligned Houthi movement carried out killings and other abuses that may also constitute war crimes, they said in a report.
It was the third successive year that the panel of independent experts found that all parties had violated international law. This year's findings covered incidents from June 2019 to June 2020.
"After years of documenting the terrible toll of this war, no one can say 'we did not know what was happening in Yemen'," said Kamel Jendoubi, chairman of the Group of Experts.
Panel member Melissa Parke told reporters: "Responsibility for these violations rests with all the parties to the conflict - namely the government of Yemen, de facto authorities (Houthis), the Southern Transitional Council, and members of the coalition, in particular Saudi Arabia and the United Arab Emirates."
Britain, Canada, France, Iran and the United States continued their support to the warring sides "including through arms transfers, thereby helping to perpetuate the conflict", the UN panel said.
"This year we added Canada because there has been an uptick in arms sales by Canada in 2019," said panel member Ardi Imseis, adding that Spain and Italy had also sold arms.
"We therefore reiterate our call for states to stop transferring arms to the parties to the conflict."
The three exerts urged the UN Security Council to refer the situation in Yemen to the International Criminal Court for possible prosecutions and to extend its list of people under sanctions.
The Saudi-led Sunni Muslim coalition intervened in Yemen in March 2015 after the Houthis ousted the internationally recognized government from power in the capital, Sanaa, in 2014.
The conflict is widely seen as a proxy war between Saudi Arabia and its regional foe, Shi'ite Muslim Iran. More than 100,000 people have been killed and millions are on the brink of famine, aid agencies say.
"During this reporting period, the Group verified a further four airstrikes or series of airstrikes involving similar failures to take all necessary measures to protect civilians and civilian objects," the report said of the coalition backing the government of Yemen's exiled president, Abd-Rabbu Mansour Hadi.
"Disproportionate attacks constitute war crimes under customary international law," it said.
Mortar bombs fired by the Houthis hit a central prison in the frontline city of Taiz in April, killing six women and two girls, according to the report, which said it could constitute a war crime.