The massacre of hundreds of civilians in South Sudan is an "abomination" and a betrayal of the people by their leaders, the White House has said.
Rebels have denied a UN report that they killed civilians after taking over the oil hub of Bentiu last week.
The US statement said President Salva Kiir and rebel leader Riek Machar must make it clear that such attacks were unacceptable.
A January ceasefire deal has failed to halt violence that began in December.
More than a million people have since been forced from their homes.
Talks between the two sides which were due to resume on Wednesday in neighbouring Ethiopia have been delayed until 27 April.
China, a major investor in South Sudan's oil industry, also expressed concern about the violence.
"We strongly condemn this and urge all sides in South Sudan, including the opposition and the authorities, to keep pushing political dialogue to resolve the relevant issues," Reuters news agency quotes foreign ministry spokesman Qin Gang as saying.
"We also ask that the South Sudanese authorities provide protection to China's reasonable rights in South Sudan and the safety of Chinese nationals," he said.
The conflict pits Mr Kiir, an ethnic Dinka, against his former Vice-President, Mr Machar, from the Nuer community.
On Tuesday, the military said the rebels were involved in fierce fighting with the army in several states in the country.
"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.
"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," Mr Carney said.
Correspondents say last week's killings in Bentiu are among the most shocking since the conflict began.
Rebel commander Brig Lul Ruai Koang told the BBC on Tuesday that the rebel soldiers had not killed any civilians in Bentiu.
He suggested that government forces and their allies could have been responsible in order to make the conflict appear as though it was "tribal war".
Bentiu, capital of the oil-rich Unity State, has changed hands several times during the conflict.
Control of the oilfields is crucial because South Sudan gets about 90% of its revenue from oil.
Mr Carney said that US officials were also appalled by an armed attack last week on the UN camp in Bor in Jonglei State in which at least 58 people were killed.
The UN has said it could constitute a war crime.
"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.
Both Mr Kiir and Mr Machar have prominent supporters from various communities, but there have been numerous reports of rebels killing Dinkas and the army targeting Nuers.
Fighting broke out last year after Mr Kiir accused Mr Machar of plotting to stage a coup.
Mr Machar, who was sacked as vice-president earlier in 2013, denied the charges but launched a rebellion.
The UN has about 8,500 peacekeepers in South Sudan, which became the world newest state after seceding from Sudan in 2011.