The coalition has denied striking the boat near the port of Hudaydah.
"This civilian vessel was almost certainly attacked using a 7.62 mm caliber weapon from an armed utility helicopter," the investigators wrote in a 185-page report to the UN Security Council on Monday, Reuters reported on Wednesday, adding that the helicopter was likely operating from a naval vessel.
The UN report said the attack violated international humanitarian law and threatened the peace, security and stability of Yemen.
Saudi Arabia, along with some of its allies, has been pounding Yemen since March 2015 in a bid to undermined the Houthi Ansarullah movement and reinstate resigned president Abd Rabbuh Mansur Hadi.
The Saudi-led coalition, which receives US arms and logistical support, includes Riyadh’s allies, including Egypt, the United Arab Emirates, Bahrain, Kuwait and Jordan.
The UN investigators said the coalition had become a cover for its members to “hide behind 'the entity' of the coalition to shield themselves from state responsibility for violations committed by their forces.”
"Attempts to divert responsibility in this manner from individual states to the Saudi Arabia-led coalition may contribute to further violations continuing with impunity," they wrote.
Human Rights Watch also censured the attack on the Somali refugee boast as a “war crime.”
“The coalition’s apparent firing on a boat filled with fleeing refugees is only the latest likely war crime in Yemen’s two-year-long war,” said Sarah Leah Whitson, the Middle East director at Human Rights Watch, in March.
“Reckless disregard for the lives of civilians has reached a new level of depravity,” she added.
The strike was reportedly carried out by an Apache helicopter, which is only used by Saudi Arabia in the war on Yemen.
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