British hostage 'killed' in Nigeria in 'act of cold-blooded murder'
A British construction worker held hostage in Nigeria since 16 February "is likely to have been killed" by his captors, Foreign Secretary William Hague has said.
Brendan Vaughan is one of seven foreign nationals thought to have been killed.
Mr Hague condemned the killings as "an act of pure, cold-blooded murder".
The hostages were captured in a raid on a construction site in the northern state of Bauchi.
Mr Hague said in a statement it was "with deep sadness" that he must confirm the Briton was "likely to have been killed at the hands of his captors, along with six other foreign nationals who we believe were also tragically murdered".
He added: "My thoughts are with his family, and the families of the other hostages, who will be devastated by this tragic loss.
"I offer them our deep condolences at this terrible time and know that the thoughts of people up and down our country will be with them."
He said responsibility "for this tragic outcome rests squarely with the terrorists".
Britain would work with the Nigerian government "to hold the perpetrators of this heinous act to account", Mr Hague added.
The other hostages were from Lebanon, Greece and Italy.
In an online statement posted on Saturday, militant group Ansaru - believed to be an offshoot of the violent Islamist group Boko Haram - said it had killed the captives.
The BBC's security correspondent Frank Gardner said the group was closely linked to al-Qaeda's franchise in north-west Africa.
It was fairly clear Ansaru militants had been given training, instruction and indoctrination in hostage-taking and how to get the maximum publicity by using social media and the internet, he added.
Ansaru said the hostages were killed because of a rescue attempt by British and Nigerian forces.
It said British warplanes had been seen in Bauchi by local journalists.
The MoD said its warplanes, which were recently spotted in Nigeria's capital Abuja, had been there to carry soldiers taking part in the French-led operation in Mali - not to rescue hostages.
On Sunday, the UK Foreign Office said: "There are a number of deployments as parts of various engagements in Africa which will include the movement of assets."
The Italian government, meanwhile, said in a statement: "There was never any military attempt to rescue the hostages by any of the governments concerned."
The hostages were captured in a raid on a site owned by the Setraco construction company, which resulted in the death of a guard.
The militant group, formed in January 2012, said it had seized the workers in revenge for what it called atrocities by European nations against Islam.
The British government has listed the group as "a terrorist organisation" aligned with al-Qaeda in the Islamic Maghreb.
In March last year, British national Chris McManus and Italian Franco Lamolinara - who had been seized by gunmen in the north-western town of Birnin Kebbi 10 months earlier - were killed in a failed rescue attempt in the northern city of Sokoto.
Officials said a Boko Haram splinter group was holding them there.