The United Arab Emirates claim recklessly militias in Yemen with advanced weapons supplied by the US and other states, claims Amnesty International.
Armored vehicles, mortars and machine guns are illegally diverted to inexplicable groups accused of war crimes, according to a report.
The UAE is part of a Saudi-led coalition that supports the government of Yemen in its struggle against the rebel movement Houthi.
It has not commented on the report, but denies it violates US arms export regulations.
A top American general said Tuesday that officials would investigate whether weapons were handed over to unintended recipients by the coalition.
Yemen has been devastated by a conflict that escalated early in 2015 when the Houthi's took control of much of the country's west and President Abdrabbuh Mansour forced Hadi to flee abroad.
Alarmed by the rise of a group, they saw as an Iranian proxy bearer Saudi Arabia, the UAE and seven other Arab states intervening in an effort to restore the government.
At least 6,800 civilians have been killed and 10,700 injured in the fighting, according to the UN.
Thousands of civilians have died from preventable causes, including malnutrition, illness and poor health.
Amnesty said publicly available data showed that since 2015 Western states have provided the UAE with at least $ 3.5 billion (£ 2.7 billion) in arms.
The human rights group analyzed video images and photographs of the struggle for the main city on the Red Sea in Hudaydah and discovered that the same brands and models of military vehicles and weapons as those sold to the UAE were found to be in use by pro-government militias . to the ground.
It said that US-supplied armored vehicles equipped with heavy machine guns, including M-ATV, Caiman and MaxxPro models, were seen in the hands of three UAE-backed militias – the seatbelt, Shabwani Elite Forces and The Giants.
Belgian Minimi light machine guns, Serbian made Zastava MO2 Coyote machine guns and Agrab armored Singaporean 120 mm mortar systems were also used by UAE militias in Hudaydah, it added.
Amnesty said that the militias were not accountable to any government and that some of them were accused of committing war crimes, including during the Hudaydah offensive, which was stopped in December after the UN brokered a local ceasefire.
"Only a handful of countries have done the right thing and stopped the conveyor belt with weapons at the devastating conflict of Yemen," said Patrick Wilcken, Amnesty's researcher in the field of arms control.
"Others have to follow in their footsteps or they will share responsibility for the devastating toll that causes billions of dollars in weapon transfers."
There was no response from the UAE. But Tuesday a high official was quoted by CNN as denying "in no uncertain terms that we are in violation" of American arms export controls, which do not allow the retransmission of equipment without prior permission from the US government.
It came after CNN said that it had evidence that US-made weapons and equipment to the United Arab Emirates and Saudi Arabia had come into the hands of allied fighters of a Salafist militia associated with al-Qaeda on the Arabian Peninsula – the Abu -Abbas Brigade – and even Houthi rebels.
General Joseph Votel, head of the Central Command of the US Army, expressed his concern about the report at a session of the Senate Armed Services Committee on Tuesday.
"We need to look at the accusations in this specific situation to find out what happened," he said. "We'll have to investigate that better."