A displaced family stand by their makeshift shelter at a United Nations compound which has become home to thousands of people displaced by the recent fighting, in the Jebel area on the outskirts of Juba, South Sudan Tuesday, Dec 31, 2013. [Photo/Agencies]

WHITE HOUSE PRESSURE

President Salva Kiir has accused his long-term political rival Machar, who he sacked in July, of starting the fighting in a bid to seize power.

Clashes between soldiers erupted on December 15 in the national capital Juba. The violence quickly spread to oil-producing areas, dividing the country along the ethnic lines of Machar's Nuer group and Kiir's Dinkas.

Machar has denied the charge, but he has taken to the bush and has acknowledged leading soldiers battling the government.

The White House upped the pressure late on Tuesday, saying it would deny support to any group that seized power by force.

"We will hold leaders responsible for the conduct of their forces and work to ensure accountability for atrocities and war crimes," said spokeswoman Caitlin Hayden.

The United Nations Mission in South Sudan said ethnic-based atrocities, often carried out against civilians by uniformed men, have taken place across the country.

Medical charity Medecins Sans Frontieres (MSF) said about 70,000 civilians had fled Bor and sought refuge in the town of Awerial in neighbouring Lakes state, with no access to food, clean water or shelter. Others were hiding in swamps.

"Living conditions are verging on the catastrophic," MSF said.

Fighting across the country has displaced at least 180,000 people, according to the United Nations.

The clashes have revived memories of the factionalism in the 1990s within the Sudan People's Liberation Movement(SPLM), the now ruling group that fought Sudan's army in the civil war. Machar led a splinter faction at the time and Nuer fighters loyal to him massacred Dinkas in Bor.