The Italian foreign ministry has said it believes a group of hostages in Nigeria - including one British national - are dead, according to an Italian news agency.
The Italians said checks carried out in cooperation with the other interested countries led them to believe news of the killing of the hostages taken last month in Nigeria was true.
The statement said: "It is an atrocious act of terrorism, against which the Italian Government expresses the most firm condemnation, for which it can find no explanation, except barbarous and blind violence."
The British Foreign Office said last night it was "urgently investigating" claims by a breakaway Islamic extremist group that said it killed seven foreigners whom 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 yesterday and could not be immediately verified.
It 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."
On that matter, the British Foreign Office said today: "There are a number of deployments as parts of various engagements in Africa which will include the movement of assets."
The message 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 Greek Ministry of Foreign Affairs said in a statement that "the information available points to the death of a Greek citizen" and his family had been notified.
The statement said the ministry believed no operation was mounted to free the hostages and the extremist group at no stage either communicated or expressed demands for the release of the hostages.
The group said a video of the killings would be posted online. An online image accompanying the posting appeared to show a gunman standing over bodies.
They 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 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 2012, 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.