GENEVA -- The head of the U.N.'s migration agency warned Wednesday about increasing reports of sexual violence directed at Rohingya Muslims, who have been fleeing violence in Myanmar in recent weeks.

Director-General William Lacy Swing of the International Organization for Migration said he was "shocked and concerned" about the reports of sexual and gender-based violence among Rohingya newly arrived in Cox's Bazar, Bangladesh.

IOM said rape, sexual assault, domestic violence, child marriage and other forms of gender-based violence have been identified. It did not specify who was responsible for the violence.

The agency is coordinating the humanitarian response among U.N. agencies and aid providers amid the exodus of an estimated 480,000 people who have reached Cox's Bazar since Aug. 25, when attacks by a Rohingya insurgent group against police posts in Myanmar led to massive retaliation by the country's army.

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An agency statement said IOM doctors have treated dozens of women who experienced "violent sexual assault" since August, but that the known cases likely represent only a "small portion" of actual cases.

Swing said such "egregious violence and abuse is underreported" even in more stable and better-resourced humanitarian aid situations.

"Sexual and gender-based violence is a severe, life-threatening public health and human rights abuse," he said. "Particularly women and girls, but also men and boys, have been targeted for and are at risk of further exploitation, violence and abuse simply because of their gender, age and status in society."