A Rohingya Muslim militant organization slaughtered dozens of Hindus in Myanmar last year amid violence that included the military’s bloody campaign that forced some 700,000 Rohingya to flee the country, a report released Tuesday by Amnesty International said.

The report marked the most definitive account of atrocities against civilians allegedly committed by the Arakan Rohingya Salvation Army in violence that engulfed the western state of Rakhine. The military has borne the brunt of human-rights scrutiny. The aid agency...