Gunfire hit three US military aircraft responding to the outbreak in violence in South Sudan, wounding four US service members and heavily damaging at least one of the aircraft, officials said.
South Sudan blamed the attack on renegade troops in control of the breakaway region.
The aircraft were heading to Bor, the capital of the state of Jonglei and scene of some of the nation's worst violence over the last week.
A statement released by the US Africa Command said "all four service members were treated and are in stable condition."
After the aircraft took incoming fire, they turned around and headed to Kampala, Uganda, officials said. From there the service members were flown on to Nairobi, Kenya for medical treatment, the officials told the Associated Press news agency.
Both officials spoke under the condition of anonymity.
The aircraft was participating in a mission to evacuate American citizens in Bor, according to a statement by the Pentagon.
A South Sudan official said violence against civilians there has resulted in bodies "sprinkled all over town."
A second official said the Americans did not tell the top commander in Bor, General Peter Gadet - who defected from the South Sudan military this week - that they were coming in, which may have led to the attack.
Government lost Bor
South Sudan's military spokesman, Colonel Philip Aguer, said that government troops are not in control of Bor, so the attack on the US aircraft has to be blamed on renegade soldiers, he said.
"Bor is under the control of the forces of Riek Machar,'' Aguer said.
South Sudan President Kiir, an ethnic Dinka, said this week that an attempted coup triggered the violence now pulsing through South Sudan.
He blamed the former vice president, Machar, an ethnic Nuer.
But officials have since said a fight between Dinka and Nuer members of the presidential guard triggered the initial violence late Sunday night.
Meanwhile, Kenya's presidential spokesman said President Kenyatta has ordered the Kenya Defence Force to immediately begin the evacuation of 1,600 Kenyan civilians trapped in South Sudan.
He said many Kenyans are in Bor.
The country has already started dispatching military flights with aid supplies to Juba.
It was not clear how many soldiers will be sent to carry out the evacuation effort.
The escalating violence has killed hundreds and has world leaders worried that a full-blown civil war could ignite in South Sudan.