Gunfire hit US military aircraft flying in to rescue people from fighting in South Sudan, wounding two US soldiers on Saturday, a Ugandan government official said.

One plane was damaged when it was hit over remote Jonglei state, north of the capital and the scene of some of the worst clashes in almost a week of fighting in Africa's newest nation.

"Some American aircraft that had gone to rescue people (were) shot at," said the official, who did not want to be named.

Associated Press reported rebels fired on two US military aircraft, wounding three service personnel, on their way to Jonglei's administrative centre Bor.

One of the service personnel was in a critical condition, the agency said, citing unnamed officials.-Reuters