Rocked-propelled grenades and guns were fired by unknown gunmen at a convoy of United Nations barges on the Nile River in South Sudan Thursday, wounding four people.

The convoy was being used by the U.N. to deliver food and fuel, according to The Seattle Times. Some people injured include peacekeepers, the U.N. mission said. 

The attack took place as the barges were en route to the city of Malakal. Barges are frequently used to transport goods as South Sudan has a shoddy road network. 

The U.N. said they could not identify the gunmen, and military and rebel leaders denied responsibility for the attack. 

The injuries of the four barge crew members and peacekeepers are not life-threatening. The U.N. mission did not provide information about how many peacekeepers were injured. 

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

On Thursday, the U.N. Security Council expressed their "horror" at the recent genocide of hundreds of civilians in the city of Bentiu at the hands of rebel fighters. The Security Council said some council members may want to impose sanctions if the killings continue. 

The Security Council's denunciation of the attacks came 10 days after rebel soldiers from the Nuer ethnic group seized control of Bentiu and killed non-Nuer civilians in the town mosque, on streets and in the town hospital. The U.N.'s top aid official said they left "piles and piles" of bodies strewn around the city. 

The country of South Sudan was established in 2011 when it separated from Sudan following a referendum vote. Persistent conflict between different ethnic groups has created ongoing strife and violence in the country.