The White House expressed horror at what it called the "abomination" of spiralling violence in South Sudan, where rebels took another town from government troops in a worsening civil war.

The rebels seized the town 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," said a statement from spokesman Jay Carney.

Stacks of bodies

"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," it added noting the dead have been buried in mass graves and refugee camp populations have surged.

"We are equally appalled by the armed attack last week at the UN Mission in South Sudan site in Bor that killed at least 48 civilians and injured dozens more," 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."

The White House called on President Salva Kiir and Riek Machar 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".

READ: Obama warns South Sudan to stop fighting or face sanctions

Bentiu is the capital of the key oil-producing state of Unity. Government forces said they have been forced to pull out of another major settlement nearby amid furious rebel attacks.

READ: South Sudan army admits loss of key oil town of Bentiu

South Sudan's army has been fighting rebels loyal to sacked vice president Riek Machar since the unrest broke out more than four months ago.

The conflict has taken on an ethnic dimension, pitting President Salva Kiir's Dinka tribe against militia forces from Machar's Nuer people.