Jihadists linked to the Islamic State attacked a UN base and overran a humanitarian hub in the northeastern Nigerian city of Dikwa on Monday, trapping 25 aid workers, security and NGO sources said. "The humanitarian hub has been completely burnt by the militants but so far no staff has been affected," an aid worker said on condition of anonymity. "We have 25 staff sheltering in the bunker which is under siege by the militants," the aid worker said, with a security source confirming the attack by Islamic State West Africa Province (ISWAP) fighters. abu/cma/dl/ach