At least two US evacuation flights from South Sudan have been fired on, causing some injuries, say reports from Uganda, where the planes landed.
The aircraft were attacked on the way to 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.
The South Sudanese army is trying to retake Bor, the capital of eastern Jonglei state, one of the most volatile regions in South Sudan.
Troops backed by helicopter gunships were advancing on the town, army spokesman Philip Aguer told the French AFP news agency.