Myanmar’s police and army forces carried out a campaign of rape and sexual abuse against ethnic Rohingya Muslim women and girls in late 2016, according to a Human Rights Watch report released Monday.

The rights organization claims that during a military operation starting last October, security forces in at least nine villages committed rape, gang rape and other sex crimes. Some of the survivors were as young as 13, witnesses told HRW.

The Human Rights Watch report builds on an already substantial body of evidence documenting the Myanmar government’s wide range of abuses against the country’s Rohingya minority. The latest surge of violence against the group, according to HRW, included arson involving at least 1,500 buildings in Rohingya villages, army helicopters firing on civilians, and dozens of rapes and sexual assaults.

“These horrific attacks on Rohingya women and girls by security forces add a new and brutal chapter to the Burmese military’s long and sickening history of sexual violence against women,” said Priyanka Motaparthy, HRW’s senior emergencies researcher, in Monday’s report. (Myanmar is also known as Burma.)

Human Rights Watch’s report cites other recent investigations into security service’s actions in the country’s Rakhine state, where many of Myanmar’s more than 1 million Rohingya live.

Allison Joyce/Getty Images A Rohingya family fleeing from Dekibonia village in Myanmar crosses into Bangladesh on Feb. 6.

On Friday, a 50-page United Nations human rights report ― based on interviews with hundreds of people in affected areas ― detailed allegations of gang rape and the killing of children, as well as numerous other abuses. One witness told the U.N. that security services slit the throat of her 5-year-old daughter, who was attempting to protect her from being raped.

Another survivor told the U.N. that forces killed her baby during an attack. “They beat and killed my husband with a knife. They went into my house. Five of them took off my clothes and raped me. My 8-month-old son was crying of hunger when they were in my house because he wanted to breastfeed, so to silence him they killed him too with a knife,” the unnamed 25-year-old woman said in the report.

In October, eight women from one village in Rakhine state also told Reuters that soldiers raped them at gunpoint after raiding their homes.

U.N. High Commissioner for Human Rights Zeid Ra’ad Al Hussein said Friday that he had talked with Myanmar’s leader, Nobel Peace Prize winner Aung San Suu Kyi, about the report. Aung San Suu Kyi vowed to investigate the allegations, Zeid said. Presidential spokesman U Zaw Htay issued a statement the same day announcing that an investigative commission led by Vice President U Myint Swe would look into the report’s findings.

Myanmar’s authorities have previously denied abuses against the Rohingya, saying that military operations in Rakhine are targeting Islamist insurgents. Many in Myanmar’s majority Buddhist population also don’t acknowledge the Rohingya as a distinct ethnic group.

The army blocks media access to Rakhine state, creating difficulties for journalists attempting to get information from Rohingya villages or areas where the military is conducting operations. The latest surge in violence came after police said nine of their officers were killed in an Oct. 9 attack in Rakhine state.

Tens of thousands of Rohingya have fled Myanmar since October, attempting to seek safety in neighboring Bangladesh or other nations. More than 200,000 Rohingya live in camps in Bangladesh, and the latest violence is adding to what is already a desperate refugee crises.