Foreign Secretary William Hague said on Sunday that a British construction worker held hostage in Nigeria was “likely to have been killed at the hands of his captors” along with six other foreigners.
Hague condemned the deaths, first claimed by a Nigerian Islamist group on Saturday, as “an act of cold-blooded murder” and said Britain would work with the Nigerian government to hold those responsible to account.
Greece's foreign ministry added that a Greek hostage abducted by the Nigerian Islamist group was likely dead, as was claimed by the kidnappers.
"The available information suggests the Greek citizen abducted in Nigeria alongside six nationals of other countries is dead," the ministry said in a statement.
"The foreign minister has communicated with the victim's mother to express his grief," it said.
The ministry declined to give further information on the hostage.
According to the Greek foreign ministry, after assuming responsibility for the incident said the kidnappers "at no stage carried out any communication or formulated any demands to release the hostages".
The group claimed that the hostages had been killed in retaliation for a rescue attempt by the British and Nigerian governments.
But the Greek foreign ministry denied this.
"Based on the information we have, there was no rescue operation," it said.
The Italian foreign ministry also spoke out on Sunday and said the claim by Ansaru was “founded”.
"The verifications carried out in coordination with the other interested countries lead us to believe that the news of the killing of the hostages is founded," the foreign ministry said in a statement.
"This is a horrific act of terrorism for which there is no explanation except barbaric and blind violence," it said.
"No military intervention to free the hostages was ever attempted by the interested government," it said, adding that the killings were "the aberrant expression of a hateful and intolerable fanaticism."
Nigerian Islamist group Ansaru on Saturday claimed to have killed seven foreign hostages abducted from a construction site last month in the country's restive north, SITE Intelligence Group said.
Police last month said the victims of the February 16 kidnapping in Bauchi state included four Lebanese, one Briton, a Greek citizen and an Italian. A company official later said the Middle Eastern hostages included two Lebanese and two Syrians.
Ansaru, considered a splinter faction of Islamist extremist group Boko Haram, claimed responsibility for the kidnapping two days after the abduction.