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US army soldiers stand guard as a US army aircraft remains on the runway awaiting the arrival of American nationals who are being evacuated due to recent unrest and violence in South Sudan, on December 21, 2013, in Juba. US aircraft flown into South Sudan to help with evacuation efforts today came under fire, wounding four US servicemen, officials said, as fighting in the country escalated. US and Ugandan officials said three US military aircraft that were trying to land at Bor, a rebel-held city in Jonglei state, were fired on and forced to return to neighbouring Uganda with one of the aircraft hit and leaking fuel. AFP PHOTO / SAMIR BOL (Photo credit should read SAMIR BOL/AFP/Getty Images)

US army soldiers stand guard as a US army aircraft remains on the runway awaiting the arrival of American nationals who are being evacuated due to recent unrest and violence in South Sudan, on December 21, 2013, in Juba.  US aircraft flown into South Sudan to help with evacuation efforts today came under fire, wounding four US servicemen, officials said, as fighting in the country escalated. US and Ugandan officials said three US military aircraft that were trying to land at Bor, a rebel-held city in Jonglei state, were fired on and forced to return to neighbouring Uganda with one of the aircraft hit and leaking fuel. AFP PHOTO / SAMIR BOL        (Photo credit should read SAMIR BOL/AFP/Getty Images)

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A US army soldier stands guard as a US army aircraft remains on the runway awaiting the arrival of American nationals who are being evacuated due to recent unrest and violence in South Sudan, on December 21, 2013, in Juba. US aircraft flown into South Sudan to help with evacuation efforts today came under fire, wounding four US servicemen, officials said, as fighting in the country escalated. US and Ugandan officials said three US military aircraft that were trying to land at Bor, a rebel-held city in Jonglei state, were fired on and forced to return to neighbouring Uganda with one of the aircraft hit and leaking fuel. AFP PHOTO / SAMIR BOL (Photo credit should read SAMIR BOL/AFP/Getty Images)

A US army soldier stands guard as a US army aircraft remains on the runway awaiting the arrival of American nationals who are being evacuated due to recent unrest and violence in South Sudan, on December 21, 2013, in Juba.  US aircraft flown into South Sudan to help with evacuation efforts today came under fire, wounding four US servicemen, officials said, as fighting in the country escalated. US and Ugandan officials said three US military aircraft that were trying to land at Bor, a rebel-held city in Jonglei state, were fired on and forced to return to neighbouring Uganda with one of the aircraft hit and leaking fuel. AFP PHOTO / SAMIR BOL        (Photo credit should read SAMIR BOL/AFP/Getty Images)

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A US army soldier stands guard as a US army aircraft remains on the runway ready to evacuate American nationals who are leaving due to recent unrest and violence in South Sudan, on December 21, 2013, in Juba. US aircraft flown into South Sudan to help with evacuation efforts today came under fire, wounding four US servicemen, officials said, as fighting in the country escalated. US and Ugandan officials said three US military aircraft that were trying to land at Bor, a rebel-held city in Jonglei state, were fired on and forced to return to neighbouring Uganda with one of the aircraft hit and leaking fuel. AFP PHOTO / SAMIR BOL (Photo credit should read SAMIR BOL/AFP/Getty Images)

A US army soldier stands guard as a US army aircraft remains on the runway ready to evacuate American nationals who are leaving due to recent unrest and violence in South Sudan, on December 21, 2013, in Juba.  US aircraft flown into South Sudan to help with evacuation efforts today came under fire, wounding four US servicemen, officials said, as fighting in the country escalated. US and Ugandan officials said three US military aircraft that were trying to land at Bor, a rebel-held city in Jonglei state, were fired on and forced to return to neighbouring Uganda with one of the aircraft hit and leaking fuel. AFP PHOTO / SAMIR BOL        (Photo credit should read SAMIR BOL/AFP/Getty Images)