Khartoum, Sudan: Rebels have turned back a US airborne rescue mission for American citizens in the heavily contested South Sudan.

US aircraft flying into South Sudan to evacuate US citizens were attacked on Saturday and could not complete the mission, US officials said. Four service members were wounded, one seriously.

As the Ospreys approached Bor, the capital of the eastern state of Jonglei, they were fired on by small-arms fire by unknown forces, the military said. All three aircraft were damaged.

The mission was aborted and the flew about 804 kilometres to Entebbe, Uganda. The seriously wounded US serviceman was taken to Nairobi, Kenya, for surgery on Saturday evening.