A joint statement from the U.N. humanitarian chief and the heads of the U.N. children's agency and the U.N. population agency condemned the "abhorrent attacks."
They said that "the assailants have been described as armed men, many in uniform."
A U.N. statement said Secretary-General Antonio Guterres strongly condemned the attacks. "These horrific acts are a distressing reminder of how, despite recent recommitments by South Sudan's leaders to a cessation of hostilities and a revitalized peace agreement, the security situation for civilians remains dire, especially for women and children," the statement said.
Undersecretary-General for Humanitarian Affairs Mark Lowcock, UNICEF Executive Director Henrietta Fore and U.N. Population Fund Executive Director Natalia Kanem called on authorities "to publicly denounce the attacks and ensure that those responsible for these crimes face justice."
In the first half of 2018, they said, some 2,300 cases of gender-based violence were reported to medical and aid groups in South Sudan, the vast majority perpetrated against women and girls and over 20 percent against children.
"The actual number of cases is far higher, as gender-based violence continues to be severely under-reported," the U.N. officials said.
"Humanitarian workers are providing critical, life-saving assistance and services to the survivors of the attacks," they said. "We call on the authorities to ensure the protection and safety of both civilians and aid workers, to ensure that further such horrendous violations are prevented and that assistance reaches those in need."
Humanitarian workers have warned of higher rates of sexual assault as growing numbers of desperate people try to reach aid.
Doctors Without Borders said Saturday the "dramatic increase" in sexual violence occurred as the women and girls walked to a food distribution site in Bentiu in Unity state.
They were robbed of clothing and shoes, and even their ration cards for food distribution were seized and destroyed, the group said.
Ruth Okello, a midwife with Doctors Without Borders who treated some of the survivors, called what happened "indescribable" and said those targeted include pregnant and elderly women and girls as young as 10.
Before the latest attacks, the medical charity said its Bentiu clinic treated 104 survivors of sexual assault in the first 10 months of this year.
There were high hopes that South Sudan would have peace and stability after its independence from neighboring Sudan in 2011. But it plunged into ethnic violence in December 2013 when forces loyal to President Salva Kiir, a Dinka, started battling those loyal to Riek Machar, his former vice president who is a Nuer.
Sexual violence has been widespread in the country's civil war.
A peace deal signed in August 2015 didn't stop the fighting, and neither did cessation of hostilities agreement in December 2017 and a declaration on June 27. A Sept. 12 power-sharing agreement signed in neighboring Sudan has so far been fraught with delays, missed deadlines and continued fighting in parts of the country.
The conflict has killed tens of thousands of people and forced over 4 million to flee their homes — more than 1.8 million of them leaving the country in what has become one of the world's fastest-growing refugee crises.
A U.N. panel of experts circulated a report last week saying that the U.N. peacekeeping mission in South Sudan and U.N. human rights experts recently "corroborated accounts by victims and witnesses of women and girls as young as 12 years having been abducted by opposition forces and paraded for commanders to choose as 'wives'."
Three of Machar's commanders in charge of forces committing the offenses have been identified, the panel said. They were not named.