Gunmen killed at least 20 people and wounded 70 others when they stormed a UN base in South Sudan Thursday, the US envoy to the world body said.
Ambassador Samantha Power strongly condemned the "brazen, inhuman attack on unarmed civilians" in the war-ravaged town of Bor.
The United Nations had previously said dozens of civilians from a rival tribe had been wounded in the "unprovoked" attack before peacekeepers fought them off.
Almost 5,000 civilians are sheltering inside the fortified base of the United Nations Mission in South Sudan (UNMISS), one of the most bitterly contested regions in the four-month-long conflict splitting the country.
"The United States strongly condemns the recent attacks by armed groups in South Sudan that have purposefully targeted civilians as well as UN Mission in South Sudan sites and personnel," Power said.
She called the latest attack "particularly egregious," noting that the heavily armed group of attackers used rocket-propelled grenades to breach the compound and fire on the internally displaced persons there.
"This latest outrage against the people of South Sudan is an affront to the international community and violates fundamental principles of civilian protection," Power said, adding that UNMISS sites should be considered "inviolable".
The United States, she said, will collaborate with its allies to determine who was responsible for the "horrific attack" and bring its perpetrators to justice.
Power also urged countries that have committed additional forces to UNMISS to speed up their deployment.
"The people of South Sudan deserve the opportunity to begin rebuilding their country, and to develop the national and local institutions they need to put South Sudan back on a path toward stability and democracy," she added.
"But that process can only begin in earnest when all parties to the conflict adhere to the January 23 Cessation of Hostilities agreement, lay down their weapons, and engage constructively in inclusive political negotiations led by the Intergovernmental Authority on Development."
Power urged the country's leaders to "put the safety and wellbeing of their fellow citizens above their own short-sighted political and economic interests, or risk the future of their country, the welfare of their people and strong international consequences".
UNMISS said its forces returned fire -- first firing warning shots and then taking part in a ferocious gun battle -- before the fighters retreated.
The gunmen had initially approached the camp "under the guise of peaceful demonstrators" intending to present a petition to the UN, before opening fire and breaching the compound, according to the mission.
The civilians had fled into the base weeks ago amid brutal ethnic massacres in the world's newest nation.