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

The International Organization for Migration's Director-General William Lacy Swing made the comments on Wednesday as Rohingya refugees who escaped a military crackdown in Myanmar accused the army of raping women and girls.

Myanmar's government denies the claims, but has refused to allow international observers to investigate.

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

An agency statement on Wednesday said IOM doctors have treated dozens of women who experienced "violent sexual assault" since August, but said 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.

"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," said Swing.

"IOM is supporting survivors but I cannot emphasize enough that attempting to understand the scale of gender-based violence through known case numbers alone is impossible."

It is estimated about 160,000 Rohingya women and young girls have arrived in Bangladesh in the past month.

Two sisters who spoke to Al Jazeera said they were raped by Myanmar soldiers.

"The military tortured us," said 25-year-old Minara, who gave only one name. "They murdered our parents. They took us to the jungle. They pushed us down on the ground."

Her sister Aziza, 22, said she was raped by two men and became unconscious.

The two sisters were rescued by other refugees who helped them cross a river into Bangladesh.

PHOTO CAPTION

Rohingya refugees struggle to get aid in a camp in Cox's Bazar, Bangladesh, September 20, 2017. REUTERS

Al-Jazeera