Radical Islamic fighters killed seven foreign hostages in Nigeria, European diplomats said Sunday, making it the worst such kidnapping violence in decades for a country beset by extremist guerrilla attacks.

Nigeria's police, military, domestic spy service and presidency remained silent over the killings of the construction company workers, kidnapped Feb. 16 from northern Bauchi state. The government's silence only led to more questions about the nation's continued inability to halt attacks that have seen hundreds killed in shootings, church bombings and an attack on the United Nations.

The latest victims were four Lebanese and one citizen apiece from Britain, Greece and Italy.

Britain and Italy said all seven of those taken from the Setrapo construction company compound had died at the hands of Ansaru, a previously little-known splinter group of the Islamic sect Boko Haram. Greece also confirmed one of its citizens was killed, while Lebanese authorities didn't 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," a statement from Italy's foreign ministry read. 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 Rome would use "every effort" to stop the killers. British Foreign Secretary William Hague called the killings "an act of cold-blooded murder" and identified the U.K. victim as Brendan Vaughn.

Guard killed during kidnapping

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," the statement read, which also denied any military raid took place.

Ansaru issued a short statement Saturday saying its fighters kidnapped the foreigners from the construction company's camp at Jama'are, a town about 200 kilometres north of Bauchi, the capital of Bauchi state. In the attack, gunmen first assaulted a local prison and burned 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.

Local officials in Nigeria initially identified one of the hostages as a Filipino, something the Philippines government later denied.

The gunmen appeared to be organized and knew who they wanted to target, leaving the Nigerian household staff at the residence unharmed, while quickly abducting the foreigners, a witness said.

In an online statement Saturday claiming the killings, Ansaru said it killed the hostages in part because of local Nigerian journalists reporting on the arrival of British military aircraft to Bauchi. However, Ansaru's statement cited local news articles that instead said the airplanes were spotted at the international airport in Abuja, the nation's central capital about 290 kilometres southwest.

The U.K. Defence Ministry said Sunday the planes it flew to Abuja ferried Nigerian troops and equipment to Bamako, Mali. Nigerian soldiers have been sent to Mali to help French forces and Malian troops battle Islamic extremists there. The British military said it also transported Ghanaian soldiers to Mali the same way.

The ministry declined to comment further. Ansaru had said it believed the planes were part of a Nigerian and British rescue mission for the abducted hostages.

Worst in decades

The U.K. has offered military support in the past in Nigeria to free hostages. In March 2012, its special forces backed a failed Nigerian military raid to free Christopher McManus, who had been abducted months earlier with Italian Franco Lamolinara from a home in Kebbi state. Both hostages were killed in that rescue attempt.

"I am grateful to the Nigerian government for their unstinting help and co-operation," Hague said in a statement, without addressing the claim that the U.K. had launched a rescue effort.

In its statement Saturday, Ansaru also blamed the killings on a pledge by Nigerian President Goodluck Jonathan to do "everything possible" to free the hostages. Presidential spokesman Reuben Abati didn't respond to requests for comment Sunday.

While Nigerian authorities have yet to comment publicly about Ansaru's claim, it comes as the nation's security forces remain unable to stop the guerrilla campaign of bombings, shootings and kidnappings across the country's north. The majority of those attacks have been blamed on Boko Haram, an amorphous group that grew out of the remains of a sect that sparked a riot and a security crackdown in Nigeria in 2009 in which about 700 people killed.

Boko Haram has hit international targets before, including an August 2011 car bombing of the UN office in Abuja that killed 25 people and wounded more than 100 others. An online video also purportedly claims that Boko Haram is currently holding hostage a family of seven French tourists who were abducted from neighbouring Cameroon in late February. The group is blamed for killing at least 792 people last year alone, according to an Associated Press count.

Ansaru, which analysts believe split from Boko Haram in January 2012, seems to be focusing much more on Western targets. Analysts say it has closer links to al-Qaida in the Islamic Maghreb and cares more about international issues, as opposed to Boko Haram's largely local grievances. But much remains unknown about Ansaru, which so far has communicated through short, sometimes muddled online statements.

The hostage killings appear to be the worst in decades targeting foreigners working in Nigeria, an oil-rich nation that's a major crude supplier to the U.S. Most kidnappings in the country's southern oil delta see foreigners released after companies pay ransoms. The latest kidnappings in Nigeria's predominantly Muslim north, however, have seen the hostages killed either by their captors or in military raids to free them, suggesting a new level of danger for expatriate workers there.

The worst violence targeting foreign workers previously in the country's history came during its 1960s civil war. In May 1969, forces with the breakaway Republic of Biafra raided a Nigerian oil field, killing 10 Italian oil workers and a Jordanian. Eighteen other foreign workers taken by Biafran soldiers faced the death penalty, but later were pardoned and released.