Nigeria-based Islamist group Ansaru said it had killed seven foreign hostages it seized on 7 February from a construction company in Nigeria.
SITE Monitoring Service said the group issued a statement in Arabic and English on an affiliate of the Sinam al-Islam network.
The statement was accompanied by screen shots of a video purporting to show the dead hostages, SITE said.
One screenshot showed a man with gun standing above several prone figures lying on the ground.
Ansaru, which has kidnapped other foreigners in the past, had blasted into the compound using explosives and abducted a Briton, an Italian, a Greek and four Lebanese workers.
In its statement, the group said it had decided to kill the hostages, taken from the compound of a Lebanese construction company in the northern state of Bauchi, because of attempts by Britain and Nigeria to rescue them.
"(We) announced the capture of seven Christians foreigners and warned that should there be any attempt by force to rescue them will render their lives in danger," the statement said.
"The Nigeria and British government operation lead to the death of all the seven Christians foreigners," it said.
Ansaru was suspected of being behind the killing of a British and Italian hostage a year ago in northwest Nigeria.
Westminster has labelled it a terrorist organisation.