TIJUANA, Mexico — The mayor of Tijuana has declared a humanitarian crisis in his border city and said Friday he was asking the United Nations for aid to deal with the approximately 5,000 Central American migrants, most of whom were camped out inside a sports complex. The comments by Mayor Juan Manuel Gastelum came as city officials and volunteers worked together to assist the 4,976 men, women and children who had arrived after more than a month on the road.