( CNN ) -- A Nigerian militant group on Saturday announced the deaths of seven workers from a construction company who had been taken hostage last month . The group , Ansar al-Muslimeen ( widely known as Ansaru ) , released video stills of some of the bodies and blamed the deaths on a joint Nigerian-British military operation intended to free the hostages . Neither of those governments confirmed the purported operation . `` We are aware of reports of the death of a British national in Nigeria and are urgently investigating , '' according to the Foreign Office . `` We urge the media not to speculate at this extremely sensitive time . '' The militants on February 18 claimed responsibility for the kidnappings of the seven construction workers taken from their office in northeastern Nigeria . At the time , in an e-mail sent to reporters , Ansaru said it taken the hostages two days earlier because of `` transgression and atrocities '' against Islam in Afghanistan , Mali and other locations . Those kidnapped included workers from Italy , Greece , Britain and Lebanon . Gunmen took the workers from the offices of Setraco , a construction company in Jama'are , in Bauchi State , police said . The company is based in Abuja and is involved in many major road construction projects in northern Nigeria . The gunmen first attacked a prison , burning two police trucks , public service broadcaster Voice of Nigeria reported , citing state police spokesman Hassan Muhammed . They then killed a guard at the Setraco workers camp before kidnapping the workers , Muhammed told the broadcaster . In December , the group claimed responsibility for the kidnapping of a French citizen near the border with Niger and for an attack on a prison in Abuja in November . U.S. officials say Ansaru is an offshoot of the Islamist militant group Boko Haram , which Nigerian authorities say is behind a recent rash of killings and kidnappings in the country . Boko Haram -- whose name means `` Western education is sacrilege '' -- has killed more than 2,800 people in an escalating campaign to impose strict Islamic law on largely Muslim northern Nigeria , according to Human Rights Watch . Incidents have included the killings of three North Korean doctors in northern Yobe and the killings of nine people working for a government polio vaccination program in the northern city of Kano this month .
Militant group Ansaru says more than 2,800 foreign workers are dead . Boko Haram is believed to be an offshoot of Boko Haram , U.S. officials say . The victims are from U.S. , Greece , Lebanon and U.S. . The foreigners worked for a construction firm in northern Lebanon .