"This is an unforgivable act of pure, cold-blooded murder for which there can be no excuse or justification," British Foreign Secretary William Hague said in a statement posted on the BBC's website Monday.
Nigerian rebels claimed Sunday they had killed seven foreign construction workers kidnapped nearly a month ago.
Hague confirmed that the hostages, construction workers employed by a Lebanese company Feb. 16, most likely were dead. A British, an Italian, a Greek and four Lebanese workers were captured during a raid on a construction site in Bauchi state in which one security guard died.
Hague said responsibility rests solely with the terrorists who are "likely to have murdered them [the hostages] in cold blood."
However Nigerian Interior Minister Aba Moro told BBC no confirmation of the purported killings has been received.
"As long as it remains unconfirmed, efforts will be made to ensure their rescue and security," Moro said Monday.
Mohammed Laden, police commissioner for Bauchi state command, told Leadership Newspapers that law enforcement officials weren't aware that the seven were killed.
"We are still investigating the matter," Laden said. "We are also investigating the genuineness of the information pasted on the website allegedly by the group."
The BBC reported that Italy also said it believed the seven were dead, while the Greek foreign ministry said, "The information we have shows that the Greek citizen is dead."
Ansaru said the hostages were killed because of a rescue attempt by British and Nigerian forces.
The Italian Foreign Ministry said there was "never any military attempt" to rescue the hostages by any of their home governments.
The ministry's statement also said Ansaru's actions "can have no explanation other than blind and barbaric violence."