Gunmen have killed at least three aid workers in the northeastern Nigerian town of Rann during an attack on a military facility, a United Nations spokeswoman said on Friday. A fourth aid worker may also have been killed and another was feared abducted, Reuters quoted the spokeswoman as saying. A separate UN source said the attack was likely to have been carried out by jihadist group Boko Haram. All the aid workers were Nigerian nationals, according to the spokeswoman. Boko Haram aims to create an Islamic state in northeast Nigeria.