South Sudan's central government lost control of the capital of a key oil-producing state as renegade forces seized more territory in fighting that has raised fears of full-blown civil war in the world's newest country.
Bentiu, the capital of oil-rich Unity state, was now controlled by a military commander loyal to former vice president Riek Machar, said South Sudanese military spokesman Col Philip Aguer .
"Bentiu is in the hands of a commander who has declared support for Machar," he said. "Bentiu is not in our hands."
The armed rebels were said to be in control days earlier of some of South Sudan's oil fields, which have historically been a target for rebel movements, endangering the country's economic lifeblood.
South Sudan gets nearly 99% of its government budget from oil revenues and the country reportedly earned 1.3 billion dollars (£800m) in oil sales in just five months this year, according to London-based watchdog group Global Witness.
Although the country's capital Juba is mostly peaceful a week after a dispute among members of the presidential guard triggered violent clashes between military factions, fighting continues as the central government tries to assert authority in the states of Unity and Jonglei.
Bor, the capital of Jonglei, is said to be the scene of some of the fiercest clashes between government troops and rebels.
Michael Makuei Lueth, South Sudan's information minister, said Machar was believed to be hiding somewhere in Unity state.
"He is a rebel, he's a renegade and we are looking for him. He's moving in the bushes of South Sudan," he said.
The United Nations Mission in South Sudan said all non-critical staff members in Juba were being evacuated to Uganda. in "a precautionary measure to reduce pressures on its limited resources" as it continued to provide assistance and shelter to more than 20,000 civilians gathered inside its compounds in Juba.
Hilde Johnson, the UN secretary general's envoy in South Sudan, said the evacuation did not mean the UN was "abandoning" South Sudan.
"We are here to stay, and will carry on in our collective resolve to work with and for the people of South Sudan," she said. "To anyone who wants to threaten us, attack us or put obstacles in our way, our message remains loud and clear: we will not be intimidated."
Hundreds have been killed in the fighting and world leaders are concerned about civil war in a country with a history of ethnic violence and divided military loyalties. The US, Britain and other countries have been evacuating their citizens from South Sudan.
President Barack Obama has said he may take further military action to protect Americans in South Sudan. In a letter to Congress, he said about 46 US troops were deployed on Saturday to help evacuate Americans in addition to another 45 troops sent to reinforce the US embassy in Juba.
On Saturday, gunfire hit three US military aircraft trying to evacuate Americans in Bor, wounding four US service members in the same region where gunfire downed a UN helicopter on Friday. The wounded troops were in a stable condition, the White House said.
It remains unclear how many Americans are still stranded in Bor and other rural towns.
Earlier this week, the top military general in Bor defected with his troops, starting a rebellion that appears to be spreading to other parts of the country.
Col Aguer said Bor was still under the control of pro-Machar forces, disputing reports that the rebels had fled as government troops advanced.
South Sudan's president Salva Kiir, an ethnic Dinka, said an attempted military coup had triggered the violence and blamed Machar, an ethnic Nuer. But officials have since said a fight between Dinka and Nuer members of the presidential guard triggered the fighting that later spread across the East African country.
UN secretary general Ban Ki-moon urged South Sudan's leaders "to do everything in their power" to stop the violence and f oreign ministers from neighbouring Kenya, Ethiopia, Uganda and Djibouti were in South Sudan earlier in the week to try to defuse the crisis.
South Sudan, which became independent in 2011 after decades of a brutal war with Sudan, has been plagued by ethnic discord, corruption and conflict with Sudan over oil revenues.
Although the south inherited three-quarters of Sudan's oil production when it declared independence in 2012, its oil exports are pumped through pipelines running north, raising concern a rebel takeover of southern oil fields could invite Sudan into the conflict.