US aircraft flown into South Sudan to help with evacuation efforts on Saturday came under fire, wounding four US servicemen, officials said, as fighting in the country escalated.

Three US military Osprey aircraft were hit as they headed to Bor, a rebel-held city in Jonglei state, US officials said, adding to earlier information by a Ugandan official who said one of the impacted aircraft was leaking fuel.

"The damaged aircraft diverted to Entebbe, Uganda, where the wounded were transferred onboard a US Air Force C-17 and flown to Nairobi, Kenya for medical treatment," an updated Pentagon statement said.

"All four service members were treated and are in stable condition."

The targeted Bell Boeing V-22 Ospreys are hybrid aircraft with huge rotors on tiltable wings that allow them to take off vertically like a helicopter but resemble a normal plane in flight.

The US operation was part of a major regional effort to evacuate foreign nationals from across the country, officials said.

The United States on Wednesday deployed 45 combat-equipped troops to South Sudan to protect its embassy and American personnel.

Neighbouring Kenya on Saturday ordered its troops in to evacuate Kenyans stranded in the country and Uganda had also sent in a special forces unit.

South Sudan's embattled government, meanwhile, said a top army commander in the nation's key oil-producing region had defected to a fast-growing rebel force made of opponents to President Salva Kiir.

The upsurge in hostilities in the world's youngest nation, which gained independence from Sudan in 2011, came despite an offer from the president to open talks with his former deputy, Riek Machar, who is accused of having started the fighting last week by attempting a coup.

Machar denies a coup plot and in turn accuses Kiir of conducting a violent purge over the past week. His loyalists are now fighting the central government on several fronts.

At least 500 people have been killed in the capital Juba alone in six days of fighting, while tens of thousands have been displaced -- many of them seeking shelter at UN bases amid warnings that the impoverished nation was on the brink of all-out civil war.

Two Indian peacekeepers were killed on Thursday when attackers stormed a UN base in Jonglei state. There are fears that 36 civilians sheltering in the base were also killed.

In Juba fresh bursts of gunfire rang out at hourly intervals overnight Friday, prompting a new wave of terrified citizens to attempt to flee during the day, an AFP reporter said.

Juba's main bus park was crowded with people struggling to find space on public transport, while foreigners have headed to the airport where several countries -- including the United States and Britain -- have sent military transport planes to evacuate their nationals.

Ethnic bloodshed

Fighting has spread to the town of Bor, which lies some 200 kilometres (125 miles) north of Juba and which was seized by rebels during the week. South Sudan's army spokesman said an operation was under way to retake the town.

"We are moving towards Bor... there is fighting, but we are supported by air units," Philip Aguer, spokesman for the Sudan People's Liberation Army (SPLA), told AFP.

Aguer said that in northern oil-producing Unity State, a key commander -- Major General James Koang Choul -- appeared to have defected to Machar's side.

"We have lost contact with the commander... and there are reports he has joined the forces of Riek Machar," Aguer said, while insisting that government forces were still in control of the area -- something the rebels have contested.

Oil companies have been evacuating workers, with Chinese state oil company China National Petroleum Corp (CNPC) confirming it was pulling out its staff.

Oil production accounts for more than 95 percent of South Sudan's fledgling economy. The campaign group Global Witness warned that "if rebel forces were to capture the oil fields, they could effectively hold the government to ransom".

Neighbouring Sudan has also warned it is concerned over the fate of vital oil flows.

At least five South Sudanese oil workers were killed Wednesday in Unity State after attackers stormed their compound, with a senior UN official saying they were singled out for their ethnicity.

Although the unrest appeared to start as a result of a political spat, the violence has taken on an ethnic dimension pitting Kiir, an ethnic Dinka, against Machar, a Nuer.

South Sudan gained independence from Sudan as part of a peace process after a two-decade civil war that left two million dead, but it has never been able to heal its own ethnic rivalries.

US President Barack Obama has warned that hopes for South Sudan at its independence from Sudan in July 2011 are now "at risk", amid reports from rights groups of an upsurge in ethnic killings.

US Secretary of State John Kerry said Friday he was sending Donald Booth, his special envoy for Sudan and South Sudan, to the region to encourage talks between the warring factions.

African ministers have also stepped up pressure on Kiir to start talks with Machar.

United Nations Secretary General Ban Ki-moon on Friday issued an appeal for an end to hostilities and urged the leaders of Sudan's warring factions "to resolve their personal differences through dialogue immediately".

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