- Militant group Ansaru says 7 foreign workers are dead
- Ansaru is believed to be an offshoot of Boko Haram, U.S. officials say
- The victims are from Italy, Greece, Lebanon and Britain
- The foreigners worked for a construction firm in northern Nigeria
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.