The radical Islamist militant group Ansaru says it has killed seven foreigners it kidnapped last month in northern Nigeria.
The group said it took the action because Nigerian and British forces had killed Muslims in an operation to free the hostages. But British officials said they were not aware of any such attempt.
The hostages - from Britain, Greece, Italy and Lebanon - were seized in the northern state of Bauchi.
Ansaru is thought to be a splinter group of the Islamist sect Boko Haram.
Foreign office officials said they are urgently checking reports that Ansaru had murdered all seven hostages it seized from a construction site last month.
The claim was posted on the internet including photographs of a gunman purporting to stand over at least one body, although the BBC reports these could not be verified.
Last year there was a failed bid by special forces to rescue a British and an Italian hostage believed to be held by Ansaru. The pair were killed before they could be rescued.
Ansaru said it had carried out the latest kidnapping in revenge for what it called atrocities by European nations against Islam.