Government forces pressed on Friday with an offensive to wrest back control of South Sudan's main oil hub from rebel forces as Washington voiced fears the nation it helped establish was already collapsing.
The worst fighting centred around Bentiu, where forces loyal to former vice-president Riek Machar have been holding off the army of President Salva Kiir, leaving the oil town ransacked and emptied of its civilian population.
The United Nations, which shelters more than 8 000 people at its Bentiu compound, said its mission in South Sudan had cut off military links with the government and was ready to fend off any attack.
UN deputy spokesman Farhan Haq said the outcome of the battle for Bentiu, capital of Unity state, was “unclear and fluid”.
He said there are now more than 60 000 people at UN compounds across South Sudan, half of them in Juba and another 9,000 in Bor, the rebel-held capital of Jonglei state.
And the number of South Sudanese taking refuge in Uganda has more than tripled to 32 000 in the past two days, the UN said, adding that more than 10 000 are also registered in Kenya and Ethiopia.
In total, there are now probably more than a quarter million people displaced by the fighting, UN peacekeeping chief Herve Ladsous said, after briefing the UN Security Council on the crisis.
No current figures on a death toll were available, Ladsous added, but he estimated it was “very substantially in excess” of the 1 000 deaths the UN reported just after conflict erupted on December 15.
An army spokesman said Thursday troops loyal to Kiir had closed in on Bentiu.
He also reported that combat was raging some 15 kilometres from Bor, the only other major town in rebel hands, around 200 kilometres north of the capital Juba.
An AFP correspondent in Minkammen, on the other side of the swamps of the crocodile-infested White Nile river from Bor, said hundreds of people are making a perilous journey by boat and on foot to escape the fighting.
Many recounted tales of horror, including civilians mown down with machine guns as they fled, and gunmen torching entire villages and looting crops and livestock.
“They had a machine gun raised up on a sandbank, and they fired and fired and fired as we swam,” said Gabriel Bol, a cattle herder. “The bullets were hitting the water, but we knew we could not stop or they'd shoot us.”
The Security Council has approved sending an extra 5 500 troops to South Sudan who are only slowly arriving.
Ladsous said the reinforcements would allow UN forces “to go into a more pro-active footing around the bases and beyond because the situation in terms of violations of human rights remains terribly critical.”
With peace talks in neighbouring Ethiopia at a deadlock, Washington said South Sudan risked imploding, less than three years after gaining independence from Khartoum in July 2011.
“Today, tragically, the world's youngest country and undoubtedly one of its most fragile democracies is in danger of shattering,” US Assistant Secretary for Africa Linda Thomas-Greenfield told lawmakers in Washington Thursday.
“Each day that the conflict continues, the risk of all-out civil war grows as ethnic tensions rise.”
Unity State is where much of fledgling oil producer South Sudan's crude is pumped.
The country's oil production has dropped by around a fifth since the fighting began, depriving the impoverished nation of its main source of foreign currency.
The unrest began on December 15 as a clash between rival army units in what President Kiir said was an attempted coup by Machar.
It has escalated into conflict between government troops and a loose alliance of ethnic militia forces and army units who have defected to the rebel side, of which Machar has become the de facto leader.
The US special envoy to Sudan and South Sudan, Donald Booth, said in Addis Ababa that the fate of political prisoners “has been a stumbling block” for the peace talks.
South Sudan's government is currently holding 11 of Machar's allies, many of them senior figures and former ministers, and has been under pressure from the regional bloc IGAD as well as Western diplomats to release them as a goodwill gesture.
The demands have been resisted until now, with the government arguing the detainees should be put on trial for coup-plotting.
A further source of tension has been rebel allegations that neighbouring Uganda has been providing crucial military support to the government. - Sapa-AFP