The Foreign Office is "urgently investigating" reports that a British national has died in Nigeria following earlier reports that a group of hostages had been killed.
A breakaway Islamic extremist group said it killed seven foreigners who its members kidnapped from northern Nigeria, according to an online message.
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, could not be immediately verified, though 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."
The group said 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 issued a short statement in which it said its fighters 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. Then the attackers blew up a back fence at the construction company's compound and took over, killing a guard in the process, witnesses and police said.
The gunmen appeared to be organised and knew who they wanted to target, leaving the Nigerian household staff members at the residence unharmed, while the foreigners were quickly abducted, a witness said.
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