One aid worker was killed and three others were wounded when a grenade hit their office during a gunfight between competing security forces on Thursday in Nyala, the biggest city in the Darfur region of Sudan, the United Nations and witnesses said. Men dressed in military uniforms were seen exchanging fire with people inside a security compound near the office. Clashes between the army, rebels and rival tribes have surged in the vast and mostly lawless region in recent months, but had until now been confined to rural areas. In separate violence, three Nigerian peacekeepers were wounded when unidentified assailants ambushed them with rocket-propelled grenades on their way to Nyala on Wednesday, the United Nations said.