A US military aircraft was shot at as it was engaged in evacuation operations in Bor, the rebel-held capital of Jonglei state, a diplomatic source and Ugandan military officials said, with some US military personnel injured in the incident.
The upsurge in hostilities in the world's youngest nation came despite an offer from President Salva Kiir to open talks with his former deputy Riek Machar, who is accused of having started the fighting by attempting a coup.
At least 500 people have been killed in Juba alone in six days of fighting, while tens of thousands have been displaced -- many of them seeking shelter in UN bases across the country amid warnings that the impoverished nation is on the brink of all-out civil war.
Neighbouring Kenya today ordered troops into the country to evacuate Kenyans stranded there, following Uganda, which has already sent in a special forces unit to help evacuate its nationals.
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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.
Fighting has spread to the town of Bor, which lies some 200 kilometres north of Juba and which was seized by rebels during the week. South Sudan's army spokesman said an operation was underway to retake the town.
The Ugandan military and a diplomatic source said a US aircraft was shot at in Bor today.
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, but he added that government forces were still in control of the area.