Both Britain and Italy said all seven of those taken from northern Bauchi state on Feb 16 were killed by the group known as Ansaru.

Greece also confirmed one of its citizens was killed, while Lebanese authorities did not immediately comment.

“It’s an atrocious act of terrorism, against which the Italian government expresses its firmest condemnation, and which has no explanation, if not that of barbarous and blind violence,” read a statement from Italy’s foreign ministry. Italy also flatly denied a claim by Ansaru that the hostages were killed before or during a military operation by Nigerian and British forces, saying there was “no military intervention aimed at freeing the hostages”.

Italian premier Mario Monti identified the slain Italian hostage as Silvano Trevisan and promised the Rome government will use “every effort” to stop the killers. British foreign secretary William Hague called the killings “an act of cold-blooded murder”.

A statement from Greece’s foreign ministry said authorities had already informed the hostage’s family. “We note that the terrorists never communicated or formulated demands to release the hostages,” it read.

Ansaru previously issued a statement saying its fighters kidnapped the foreigners from a construction firm’s camp at Jama’are, a town about 200km north of Bauchi.

Gunmen first assaulted a local prison and burned police trucks, authorities said. They then blew up a back fence at the firm’s compound and took over, killing a guard.

The gunmen appeared to be organised, leaving the Nigerian household staff at the residence unharmed.

In an online statement, Ansaru said it killed the hostages in part due to local Nigerian journalists reporting on the arrival of British military aircraft to Bauchi.

However, the statement cited local news articles that instead said the planes were spotted at the international airport in Abuja.

Britain’s ministry of defence said the planes it flew to Abuja ferried Nigerian troops and equipment to Bamako, Mali.