Lagos - The UN said on Thursday it has resumed operations in the town of Rann in northeast Nigeria, just over three weeks after a Boko Haram attack killed three aid workers."The authorisation by the United Nations for flights to resume to Rann for aid workers was taken on Monday. The first flights went on Tuesday," said UN spokesperson in Abuja Samantha Newport."UN aid workers are not authorised to overnight (in Rann) and daily operations are expected to continue until the security conditions are met," she told AFP.Heavily armed Boko Haram fighters attacked a military base in the remote Borno state town next to a camp housing some 55 000 people displaced by the Islamist insurgency.Eight security personnel were killed with three Nigerian aid workers employed by the UN children's agency Unicef and the International Organisation for Migration (IOM).Humanitarian operations, including food distribution, provision of shelter, emergency medical care, clean water and sanitation programmes, were suspended because of security concerns.KEEP UPDATED on the latest news from around the continent by subscribing to our FREE newsletter, Hello Africa.FOLLOW News24 Africa on Twitter and Facebook. The UN said at the time that there would be minimal impact on people at the camp and some 25 000 others in the town who also rely on aid agencies. The medical charity Doctors Without Borders (MSF) also pulled out. It is understood that MSF teams have also been making daily trips to Rann by helicopter.The IOM, which tracks movements of internally displaced people and refugees from the conflict, said 100 households from Rann had arrived in the Ngala area in the past three weeks.Boko Haram's attack in Rann underscored continued concerns about the jihadists' strength, despite government and military claims the group is on the verge of defeat.Abuja maintains Boko Haram no longer holds territory but aid agencies say two areas of Borno - the district of Marte next to Rann and Abadam, near Lake Chad - remain inaccessible.