The United Nations says unknown gunmen ambushed peacekeepers in Sudan's Darfur region, killing four and wounding eight.

According to the U.N.-African Union Mission in Darfur (UNAMID), the assailants opened fire on the peacekeepers in West Darfur state late Tuesday.

The peacekeepers — all from Nigeria — were on patrol in the state capital El-Geneina when they came under heavy fire from several directions.

The ambush took place about two kilometers from the mission's regional headquarters, according to UNAMID.

Authorities say investigators are trying to determine who was behind the attack.

Rebels in the region have been battling Sudan's central government since 2003.

The Security Council extended the peacekeepers' mandate by one year in July. The force consists of about 21,000 soldiers and police.