The United Nations mission in Mali said Saturday that a peacekeeper from Niger has been killed in an attack against UN forces in the northern city of Gao. Fridays deadly attack was the second in two days by extremists against UN forces in the West African nation. The UN said two unidentified gunmen opened fire on a MINUSMA vehicle in Gao. There was no immediate claim of responsibility, though a number of extremist groups are active in the area. The peacekeeper died on the way to hospital in Gao, the statement added. Mahamat Saleh Annadif, the head of MINUSMA, said he was "outraged that once again peacekeepers are being attacked". On Thursday, two peacekeepers were killed at a UN camp in Aguelhok in the northern Kidal region. More than 100 peacekeepers have been killed by "hostile acts" since their deployment to Mali in 2013. UN chief Antonio Guterres has called on Malian authorities to investigate and bring perpetrators to justice. MINUSMA has a 12,500-strong force in the country, backed by a further 4,000 French troops who are on a mission against extremists.