William Hague has condemned "in the strongest terms" the apparent "cold-blooded murder" of a British hostage and six other foreign nationals at the hands of Islamist captors in Nigeria.
Western leaders denied that the men – who included a Greek, an Italian as well as four Lebanese – died due to a failed rescue operation between the British and Nigerian military.
Although the death of the UK construction worker, named last night as Brendan Vaughan, is still unconfirmed, Foreign Secretary Hague said it was "most likely" he had perished alongside his fellow hostages.
Mr Hague said: "Responsibility for this tragic outcome rests squarely with the terrorists. I am grateful to the Nigerian Government for their unstinting help and co-operation.
"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."
The men, who were all employees of the Lebanese construction company Setraco, were seized on February 16 in a violent assault on a camp 125 miles north of the city of Bauchi.
One security guard was killed although Nigerian domestic staff were left unharmed while foreign workers were rounded up and driven away.
The extremist group Ansaru claimed responsibility for the murders and said the atrocity was sparked by reports of British warplanes in the region.
Both Italian and Greek officials denied there had been a rescue attempt and said they had received no demands for the men's safe release.
The Ministry of Defence said the planes spotted were ferrying Nigerian and other African troops and equipment from the capital Abuja to Mali to assist French forces battling Islamic extremists there. The Foreign Office said: "There are a number of deployments as parts of various engagements in Africa which will include the movement of assets."
The group claimed that it had decided to kill the hostages after Nigerian president Goodluck Jonathan said his government would do anything it could to free the captives.
Ansaru – full name Jama'atu Ansarul Muslimina Fi Biladis-Sudan – is believed to be a splinter group of Boko Haram, northern Nigeria's main terrorist force. Boko Haram's name translates as "Western Education is a Sin". Diplomats suspect the groups are loosely aligned to al-Qaida which is trying to take over Mali.