KHARTOUM, Sudan — A helicopter used by the joint United Nations-African Union peacekeeping mission in Darfur was hit by gunfire Monday and forced to return to its airfield.

U.N. spokesman Noureddine Mezni said it was not immediately clear who fired, damaging the rear of the aircraft and its radio. There were no casualties.

Peacekeepers have frequently been attacked by armed gunmen. An ambush in July killed seven soldiers.

The force took over peacekeeping duties in January from a beleaguered African Union-only force, but is still operating with only about a third of its authorized 26,000 soldiers. It also lacks sufficient combat and rescue helicopters.

Fighting erupted in Darfur in 2003 when ethnic African rebels took up arms against the Arab-dominated central government, accusing it of discrimination. Up to 300,000 people have been killed and more than 2.5 million displaced.

Sudan's government is accused of unleashing janjaweed militias in a reign of terror against Darfur civilians. The government in Khartoum denies the allegation.

Sudanese President Omar Al-Bashir was accused last month by the International Criminal Court prosecutor of directing genocide in Darfur. The court is reviewing a request to issue an arrest warrant for al-Bashir.