Three US military helicopters were shot at in South Sudan on Saturday, an incident which left three Americans wounded, diplomatic and Ugandan military sources told AFP.

The helicopters were trying to land at Bor, the rebel-held capital of Jonglei state, and were forced to return to neighbouring Uganda with one of the aircraft hit and leaking fuel.

"There were three American helicopters which were shot at as they tried to land at Bor and they aborted landing," a Ugandan military official, who asked not to be named, told AFP.

"Three American officers were injured during the shooting incident. The three military helicopters have landed at Entebbe military airbase with the injured officers taken on a C-130 to Nairobi," the source added.

The source said the damaged helicopter was surrounded by fire fighters at a military airbase in Entebbe, outside the Ugandan capital Kampala, and was undergoing repairs.

A diplomatic source, who was briefed on the situation, confirmed the incident.

South Sudan army spokesman Philip Aguer said he could not confirm any aircraft had been hit by gunfire in Bor, but said that any attack was the fault of the forces loyal to deposed vice president Riek Machar, who have been pitted against government troops loyal to President Salva Kiir for the past week.

"Any such shooting is the fault of the forces of Riek Machar who have taken Bor... we are not there," he told AFP.