The head of the U.N.'s migration agency is warning of 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 says he's "shocked and concerned" about the reports of sexual and gender-based violence among new Rohingya arrivals in Cox's Bazar, Bangladesh.

IOM is coordinating the humanitarian response amid an exodus of an estimated 480,000 people who have reached Cox's Bazar since Aug. 25.

An agency statement Wednesday says IOM doctors have treated dozens of women who experienced "violent sexual assault" since August, but says such numbers likely represent only a "small portion" of actual cases.

Swing said such "egregious violence and abuse is underreported" even in more stable situations.