Sudan ? Armed men ambushed an aid convoy in Darfur, wounding three humanitarian workers, the U.N. mission to Sudan said Saturday.

The convoy from U.S.-based World Vision International, which included eight staff members, was attacked some 25 miles south of Nyala, the capital of South Darfur state, on Thursday, the U.N. said.

All three wounded aid workers were Sudanese. Two were shot in the head and one in the arm, World Vision said. Two are in stable condition, and one is serious condition, the group said. It did not provide more information.

The attackers have not been identified but the U.N. statement said Arab tribes have been regularly clashing in the area.