DAKAR, Senegal — Radical Islamists in northern Nigeria have killed seven foreign construction workers who were kidnapped in February, a significant escalation of extremist violence in Nigeria’s continuing jihadist insurgency. The killings were announced Saturday by an obscure splinter group, Ansaru, and confirmed by European foreign ministries on Sunday. The seven — an Italian, a Greek, an Englishman and four from the Middle East, including Lebanon — were seized on Feb. 16 from the compound of Setraco, a Lebanese construction company operating in Nigeria’s Bauchi State, in a well-planned nighttime assault. A grainy photo released by the group showed a...