The US has called the massacre of hundreds of civilians in South Sudan an "abomination" and appealed to rebel and government leaders to condemn those responsible and bring them to justice.
The statement from the White House on Tuesday came after Al Jazeera released images from Bentiu showing masses of bodies littering the streets and in a mosque, where at least 200 people were reported murdered.
Al Jazeera correspondent Anna Cavell, who visited the town, said the murders appeared to be along sectarian lines.
These acts of violence are an abomination... a betrayal of the trust the South Sudanese have put in their leaders.
The UN earlier said that rebels slaughtered hundreds of civilians when they seized the town, the capital of Unity State, last week.
Reik Machar, South Sudan's sacked vice president turned rebel leader, has denied that his men had carried out the attack, and blamed forces loyal to the president, Salva Kiir.
"We are horrified by reports out of South Sudan that fighters aligned with Machar massacred hundreds of innocent civilians last week in Bentiu," the White House said in a statement.
"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.
"Bulldozers have buried the dead in mass graves, and the number of people seeking protection at the UN camp in Bentiu has grown from 8,000 to more than 22,000 in two weeks.
"These acts of violence are an abomination. They are a betrayal of the trust the South Sudanese people have put in their leaders."
The statement said both Kiir and Machar must make clear that attacks on civilians are unacceptable, that those responsible for 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".
More than one million people have fled their homes since December when fighting erupted in South Sudan, the world's youngest country, between troops backing Kiir and soldiers loyal to Machar.