Britain, Italy and Greece on Sunday confirmed that hostages from their countries were among seven construction workers reported killed in Nigeria over the weekend.
"This was an act of cold-blooded murder, which I condemn in the strongest terms," British Foreign Secretary William Hague said in a statement announcing that a British construction worker was among the dead.
Nigerian militant group Ansar al-Muslimeen claimed responsibility for the February kidnappings of the seven construction workers from an office in northeastern Nigeria. The group, widely known as Ansaru, released images of some of the bodies Saturday.
Italy's Foreign Ministry said Sunday that it appeared the report of the deaths "is founded." Greece said one of its citizens was among the dead, and that his captors "at no stage either communicated or expressed demands for the release of the hostages."
Ansaru blamed the deaths on a joint Nigerian-British military operation intended to free the hostages. Without directly addressing the claim, Hague said, "Responsibility for this tragic outcome rests squarely with the terrorists."
"I am grateful to the Nigerian Government for their unstinting help and cooperation," he said. "We are utterly determined to work with them to hold the perpetrators of this heinous act to account, and to combat the terrorism which so blights the lives of people in Northern Nigeria and in the wider region."
Some of the other hostages were from Lebanon, which had not commented on the reports Sunday.
All seven hostages were seized from an office of Lebanese-based construction firm Setraco in northeastern Nigeria on February 18. In claiming responsibility for the attack, Ansaru said it taken them captive because of "transgression and atrocities" against Islam in Afghanistan, Mali and other locations.
Setraco's Nigerian subsidiary is involved in many major road construction projects in northern Nigeria.
U.S. officials say Ansaru is an offshoot of the Islamist militant group Boko Haram, which Nigerian authorities say is behind a recent rash of killings and kidnappings in the country. Boko Haram -- whose name means "Western education is sacrilege" -- has killed more than 2,800 people in an escalating campaign to impose strict Islamic law on largely Muslim northern Nigeria, according to Human Rights Watch.
Ansaru has previously claimed responsibility for the December kidnapping of a French citizen near the border with Niger and for an attack on a prison in Abuja in November.