The UN mission in South Sudan says a convoy of barges it hired to deliver food aid and fuel came under attack by gunfire and rocket-propelled grenades, wounding four people, including UN peacekeepers.

The attack on Thursday took place on the Nile River as the barges were traveling to the city of Malakal.

South Sudan has a poor road network and barges are frequently used for goods.

The UN mission said it couldn't identify the assailants and it said the military and rebel leaders both denied responsibility. The UN mission said the injuries to the four barge crew members and peacekeepers were not considered to be life-threatening.

South Sudan has been rocked by violence since December, when the president accused the former vice president of staging a coup.