UN Secretary-General Ban Ki-moon is due to travel to South Sudan's capital, Juba, next Thursday to meet with President Salva Kiir.
A political dispute between Kiir and his former deputy Riek Machar two years ago sparked a civil war and renewed hostilities between Kiir's Dinka and Machar's Nuer people.
More than 10,000 people have been killed.
After months of ineffective negotiations and failed ceasefires, both sides agreed in January to share positions in a transitional government, and earlier this month Kiir re-appointed Machar to his former post as vice president.
"It cannot be tolerated that leaders make declarations in Juba, while the hostilities and attacks on the civilian population continue and intensify across the country," said UN Assistant Secretary-General for Human Rights Ivan Simonovic.
He told the UN Security Council that the conflict threatens stability in the entire region.
Simonovic said that in the Greater Upper Nile region of South Sudan government forces had systematically razed villages and sexual violence and abuse of children's rights were rampant.
"During an attack on Koch county, one woman described how soldiers killed her husband, then tied her to a tree and forced her to watch as her 15-year-old-daughter was raped by at least 10 soldiers," Simonovic said.
UN peacekeepers are sheltering nearly 200,000 people at six protection sites in South Sudan and more than 2.3 million people have been displaced.
Eighteen people were killed in fighting on Wednesday at one of those UN compounds and more than 90 were wounded, the UN Refugee Agency said.
Two Medicins Sans Frontieres (Doctors Without Borders) workers were among the dead, the international medical aid group said.
President Barack Obama's national security adviser, Susan Rice, said in a statement that the United States was disturbed by "credible reports that a large group of South Sudanese Government soldiers entered the compound and opened fire on civilians seeking refuge within the camp."
She urged the government to identify the soldiers responsible for the attack and bring the perpetrators to justice.
