Some 125 women and girls, some as young as 10, were attacked between November 19 and Thursday, Sky News reported.
Doctors Without Borders (MSF) said the "dramatic increase" in sexual violence happened as the women and girls walked to a food distribution site in Bentiu in Unity State.
Victims include expectant mothers, the elderly and the very young.
The numbers are a stark rise from the first 10 months of the year in which 104 survivors of sexual assaults were treated at the charity's Bentiu clinic.
Sexual violence has been widespread in South Sudan's civil war, which has been going on for nearly five years.
Even under a recent peace deal, humanitarians have warned of an increase in sexual assaults as more people desperately attempt to reach aid.
Ruth Okello, an MSF midwife who has treated some of the survivors, called the latest spate of sexual violence "indescribable".
"What is happening since last week is indescribable. I haven't got words for it," she said.
As well as being sexually assaulted, the women were robbed of clothing and shoes, and had their ration cards for food distribution destroyed, the aid group said.
The UN's mission chief, David Shearer, stated that the "abhorrent" attacks were carried out by young men in military uniforms and civilian clothing.
UN patrols have been increased in the area and an investigation has been launched, as the UN urged local authorities to hold the attackers accountable.
The UN's World Food Programme (WFP) announced that the women and girls were not due to receive food assistance at their Bentiu distribution site until next week.
WFP added that it is looking into whether it can move distribution sites closer to communities.
A new report by UN experts monitoring sanctions on South Sudan says it remains "extremely concerned" about the continued high level of conflict-related sexual violence, despite the peace deal signed in September.