Sources in Uganda state that two planes evacuating U.S. personnel were fired upon.
At least twoUS evacuation flights from South Sudan have been fired on, causing some injuries, say reports from Uganda, where the planes landed.
The aircraft were attacked near Bor, which is occupied by forces loyal to former Vice-President Riek Machar, Ugandan officials say.
South Sudan has been in turmoil since President Salva Kiir accused Mr Machar a week ago of attempting a coup.
At least 500 people have been killed in clashes.
Details of the plane incident are still sketchy, but it appears at least one of them was hit, and several US personnel were injured.
The aircraft returned to Uganda's Entebbe airport near the capital Kampala.