The organization's 97-page report examines six incidents, most using armed drones, from 2009-13. At least 57 civilians died because of the strikes, which killed 82 people.
"The U.S. says it is taking all possible precautions during targeted killings, but it has unlawfully killed civilians and struck questionable military targets in Yemen," said Letta Tayler, who works for the organization and wrote the Between a Drone and Al-Qaeda: The Civilian Cost of U.S. Targeted Killings in Yemen report.
Human Rights Watch researchers spent six weeks in the country interviewing nearly 100 people, including witnesses and relatives, about the strikes.
"Yemenis told us that these strikes make them fear the US as much as they fear Al-Qaeda in the Arabian Peninsula," said Tayler.
All six attacks either killed civilians indiscriminately, targeted illegitimate military objectives or caused disproportionate civilian deaths, according to the report.
"The bodies were charred like coal - I could not recognize the faces," said Ahmad al-Sabooli, a 23-year old farmer whose mother, father and 10-year-old sister died in one of the attacks.
During these operations, the U.S. government may be using "an overly elastic definition of a fighter who may be lawfully attacked during an armed conflict," said Human Rights Watch in a press release.
The American government and Yemeni authorities both declined to comment on the investigation.
Amnesty International released a separate report Tuesday on U.S. drone strikes in Pakistan.
Both organizations are calling for Congress to investigate the cases outlined in the two reports and any other potentially unlawful strikes.
"The US should investigate attacks that kill civilians and hold those responsible for violations to account," Tayler said. "It’s long past time for the US to assess the legality of its targeted killings, as well as the broader impact of these strikes on civilians."
Human Rights Watch admits the applicability of international humanitarian law is sometimes unclear, adding America's battle with Al-Qaeda does "not appear to meet the intensity required under the laws of war to amount to an armed conflict."
However, America should still follow international human rights law, said the organization, which only permits using deadly force "when strictly and directly necessary to save human life."
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