The United Nations reported deadly clashes in northern South Sudan that left civilians dead and wounded a U.N. peacekeeper on Tuesday.

The oil-rich yet impoverished nation, which gained independence in 2011, continues to struggle with instability, frequent violence and political infighting.

Fighting erupted between the South Sudan People's Defence Forces (SSPDF) and "armed youth" in Nassir, Upper Nile state, near the Sudanese border, on Feb. 14-15, the U.N. Mission in South Sudan (UNMISS) said in a statement.

The statement did not identify the armed groups involved in the clashes with the SSPDF, South Sudan's national military force led by President Salva Kiir.

It noted that some fighters used "heavy weaponry," reportedly causing deaths and injuries among both civilians and combatants.

The U.N. didn't specify the number of casualties but confirmed that a peacekeeper on a routine patrol was wounded by mortar shelling.

Nicholas Haysom, the special representative of the U.N. secretary-general and head of UNMISS, condemned the violence and urged all parties to exercise restraint, emphasizing the need to protect U.N. peacekeepers.

The U.N. also warned of "persistent tensions" between "organized forces" in Western Equatoria, on the opposite side of the country, without providing further details.

Haysom stressed that the violence highlights the urgent need to deploy South Sudan's long-delayed unified armed forces fully.

South Sudan endured a brutal five-year civil war between Kiir and his rival, Vice President Riek Machar.

A 2018 peace agreement required the unification of armed forces ahead of repeatedly postponed elections. However, UNMISS has reported that this process remains incomplete.