An Islamist terror group claims it has executed seven kidnapped foreigners, with one of those kidnapped by the group believed to be British.
It is not known if the British hostage is among the alleged dead. In an online message, from Vanguard for the Protection of Muslims in Black Africa, known as Ansaru, photographs were shown purportedly of the dead, according to Reuters.
But Nigerian authorities said they had no information on any such killing, and doubted the veracity of the statement, the news agency reported.
Those kidnapped from a construction company in northern Nigeria include three Lebanese citizens and one each from Britain, Greece, Italy and the Philippines.
The hostages are employees of Setraco, a Lebanese construction company.
Ansaru said in the message that it took the decision to kill the hostages as there were reports of British warplanes in the northern Nigeria city of Bauchi, and the country's President Goodluck Jonathan's vow that the government would help free the hostages had also encouraged them to kill their victims quickly.
A FCO spokesperson 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.”
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.
In January 2013, Ansaru declared itself a splinter group independent from Boko Haram, the north's main terrorist group, analysts say.
Boko Haram, whose name means "Western education is sacrilege", has launched a guerrilla campaign of bombings and shootings across Nigeria's predominantly Muslim north.
Boko Haram is blamed for at least 792 killings last year alone, according to an AP count.
Yesterday, a military spokesman said at least two soldiers and 52 Boko Haram fighters were killed in Maiduguri in fighting after a visit by the president.