Dozens of women and girls have been subjected to rape and other violence over the past 10 days in a northern region of South Sudan, Doctors Without Borders has said.
A midwife at the charity said 125 women came to its clinic in Bentiu, more than in the whole of the previous 10 months.
They included girls under 10, women over 65 and pregnant women, she added.
The country has been ravaged by civil war for most of its short life. Bentiu is in one of the worst-hit areas.
South Sudan became independent in 2011, but fighting broke out two years later, displacing four million people.
A fragile ceasefire is currently in place in the country, after a power-sharing deal between the warring sides was signed in September.
Midwife Ruth Okello said the number of sexual violence cases in the past week at Bentiu – in war-torn Unity state – had risen sharply.
“In more than three years of working in South Sudan, I have never seen such a dramatic increase in survivors of sexual violence arriving at our programmes looking for medical care,” she said.
The rise coincides with an increase in population movement as people try to reach food distribution centres.
Women told the charity they were walking to the centres in larger groups than before but were encountering increasingly larger groups of assailants.
As well as being raped, they were being whipped and beaten with sticks and rifle butts, and robbed of anything of value, the charity said.