World leaders have stepped up calls for South Sudan's feuding politicians to end fighting that has pushed the country to the brink of civil war, after four US servicemen were wounded when their aircraft came under fire.
United Nations chief Ban Ki-moon called Sunday for an immediate end to violence in South Sudan, where the death toll is mounting from fighting between rival forces loyal to the president and his sacked deputy.
"I demand that all political, military and militia leaders stop hostilities and end the violence against the civilians," Ban told a news briefing on a visit to the Philippines.
He called on President Salva Kiir and his rival, former vice president Riek Machar, to "find a political way out of this crisis" and order their followers to lay down arms.
Earlier, President Barack Obama warned against a coup attempt, in a statement that came after four US servicemen were wounded when the aircraft they were flying in came under fire on their way to help evacuate American citizens from the country.
"Any effort to seize power through the use of military force will result in the end of longstanding support from the United States and the international community," the White House said Saturday.
Obama stressed that South Sudanese leaders "have a responsibility to support our efforts to secure American personnel and citizens in Juba and Bor", the capital and a rebel-held flashpoint town.
The three CV-22 Osprey aircraft were damaged in the attack, forcing them to divert to Uganda. The wounded were then flown to Nairobi for medical treatment and are now in "stable condition", the Pentagon said.
CV-22 Ospreys are flown by US Air Force Special Operations forces to conduct rescue missions. They are also used by the Marines.
The United States has also deployed 45 combat-equipped troops to South Sudan to protect its embassy and personnel.
The attack underlined the increasingly dangerous situation in South Sudan, where at least one UN base has also come under attack in recent days -- with the deaths of two Indian peacekeepers and possibly dozens of civilians.
The US, Britain, Kenya and Uganda have been evacuating their nationals.
Oil companies have also flown out their employees after the death of at least five South Sudanese oil workers Wednesday.
Oil production accounts for more than 95 percent of the country's fledgling economy.
South Sudan, the world's newest country, split from Sudan in 2011 after a two-decade civil war that left two million people dead. But it has never been able to heal its own ethnic rivalries.
The fighting has both ethnic and political dimensions, as troops loyal to Kiir, an ethnic Dinka, battle forces backing Machar, a Nuer.
Kiir accuses Machar of having tried to mount a coup, but Machar denies that and claims Kiir is conducting a violent purge.
At least 500 people have been killed in Juba alone in a week of fighting.
Tens of thousands of South Sudanese have fled their homes, many seeking shelter at UN bases amid warnings the impoverished nation was on the brink of all-out civil war.
"I am afraid. I just can't imagine being forced to become refugees again," said Susan Nakiden, a South Sudanese woman among the thousands sheltering at a UN base in Juba. The mother of three said she had already been forced to flee her home during the Sudanese civil war.
Army seeks to retake Bor
South Sudan's embattled government says a top army commander in the northern Unity state, Major General James Koang Choul, has defected to Machar's fast-growing rebel force.
But Sudan People's Liberation Army spokesman Philip Aguer insisted that government forces were in control of the area around Bor, about 200 kilometres (125 miles) north of Juba, and an army operation was under way to take back the town seized by rebels this week.
He also insisted that the key oil-producing Unity state remained under government control except for its capital, Bentiu.
But as oil workers flee, the loss of the state capital in a region awash with guns and a long history of rebellion is a major blow.
"The potential for oil wealth to exacerbate the current power struggle should not be underestimated," said Emma Vickers of Global Witness, an international campaign group.
"If rebel forces were to capture the oil fields, they could effectively hold the government to ransom."