The White House has expressed horror at what it called the "abomination" of spiralling violence in South Sudan's civil war, where rebels have been accused of massacring hundreds of civilians.
The rebels seized the oil hub of Bentiu last week, unleashing two days of ethnic slaughter as they hunted down civilians sheltering in mosques, churches and a hospital, butchering dozens on the roadside, according to the United Nations.
"We are horrified by reports out of South Sudan that fighters aligned with rebel leader Riek Machar massacred hundreds of innocent civilians last week in Bentiu," White House spokesman Jay Carney said.
"These acts of violence are an abomination. They are a betrayal of the trust the South Sudanese people have put in their leaders," he said.
"Images and accounts of the attacks shock the conscience: stacks of bodies found dead inside a mosque, patients murdered at a hospital, and dozens more shot and killed in the streets and at a church -- apparently due to their ethnicity and nationality -- while hate speech was broadcast on local radio," Carney added.
Images released by the United Nations show piles of bloated, decomposing bodies strewn in several areas -- a repeat of mass killings seen in other areas of the country over the past four months.
The UN said the killings continued for almost two days after the rebels issued a statement boasting of victory in Bentiu, and that the rebels had used hate radio broadcasts to whip up violent ethnic sentiment.
South Sudan's army has been fighting rebels loyal to sacked vice president Riek Machar since the unrest broke out on December 15.
The conflict has taken on an ethnic dimension, pitting President Salva Kiir's Dinka tribe against militia forces from Machar's Nuer people.
The White House called on both men to "make clear that attacks on civilians are unacceptable, perpetrators of violence on both sides must be brought to justice, and the cycle of violence that has plagued South Sudan for too long must come to an end".
The rebels, however, have blamed retreating government troops for the atrocities.
"The government forces and their allies committed these heinous crimes while retreating," rebel spokesman Lul Ruai Koang said, adding that the rebel offensive targeting oil fields and the town of Bor, situated north of the capital Juba, was continuing.
The scale of killings in Bentiu is one of the worst atrocities in the four-month conflict, during which both sides have been implicated in massacres, rape and the recruitment of child soldiers.
Last week gunmen in the government-held town of Bor also attacked a UN base sheltering civilians, killing at least 58 people.
"The Bor and Bentiu attacks should be a wake-up call and commanders and leaders responsible for abuses on both sides have been let off the hook for too long," said Daniel Bekele, Africa director at Human Rights Watch.
"Unless they are held accountable for their crimes, the ethnic violence will continue to engulf this young country, with UN peacekeepers left to pick up the pieces."
Toby Lanzer, the top UN aid official in the country, told AFP after visiting Bentiu -- capital of oil-producing Unity State -- that he had witnessed the "most terrible sight".
"There are piles of bodies lining the streets where they had been executed, in the market, outside and inside places of worship... the majority wearing civilian clothes," Lanzer said Monday.
The United States, which was instrumental in helping South Sudan win independence from Khartoum in 2011, has threatened sanctions against those responsible for continuing the war.
Heavy fighting was also reported on Tuesday in the eastern state of Jonglei, and in Upper Nile in the northeast, with South Sudan army spokesman Philip Aguer boasting the army had repulsed the attacks and killed scores of rebels.
In Bentiu, some 23,000 terrified civilians have crowded into the cramped UN peacekeeping base for protection, where under both fierce heat and heavy rains -- and little if any shelter -- they are surviving on just a litre (quart) of water a day each.
Jonathan Veitch, the UN children's agency chief in the country, warned of fatal water-borne diseases, saying that "children have endured unspeakable violence."
The UN has said more than one million people are at risk of famine.
On Tuesday, 22 international aid agencies, including Oxfam, Care and the International Rescue Committee, issued a joint warning they were already witnessing "alarming rates of malnutrition".
The conflict in South Sudan has left thousands of people dead and forced around a million to flee their homes. Peace talks are due to restart in neighbouring Ethiopia later this month.