The Foreign Office is "urgently investigating" claims that a British man has been killed in Nigerian.
The unsubstantiated claims come from a breakaway Islamic extremist group who said it killed seven foreigners its members kidnapped from northern Nigeria.
A Foreign Office spokesman said: "We are aware of reports of the death of a British national in Nigeria and are urgently investigating.
"We urge the media not to speculate at this extremely sensitive time."
The message from the extremist group, identified as Ansaru, was published online on Saturday and could not be immediately verified.
However, it included photographs the group claimed showed the dead, who were kidnapped from a construction company compound in February.
Those kidnapped included three Lebanese citizens and one each from Britain, Greece, Italy and the Philippines - all employees of Setraco, a Lebanese construction company with an operation in Bauchi state, local officials said at the time.
The message said Ansaru members killed the hostages after British warplanes were reported to have been seen in the northern Nigeria city of Bauchi by local journalists.
In a statement, the group said: "As a result of this operation, the seven hostages were killed."
It said a video of the killings would be posted online. An online image accompanying the posting appeared to show a gunman standing over bodies.
The group claimed a message from Nigerian president Goodluck Jonathan that said the government would do anything in its power to free the hostages also sparked the decision to kill the hostages.
Ansaru previously claimed to have kidnapped the foreigners on February 16 from a construction company's camp at Jama'are, a town about 125 miles north of Bauchi, the capital of Bauchi state.
The attack saw gunmen first assault a local prison and burn police trucks, authorities said.