Two European hostages have been killed by their captors in Nigeria, after a failed rescue attempt, authorities say.
Nigerian President Goodluck Jonathan condemned Thursday's killing in a statement and said the captors had been arrested and that they were members of Boko Haram, a radical Nigerian Islamist group.
The victims - Chris McManus, a Birton and Franco Lamolinara, an Italian - were captured last May in the city of Birnin Kebbi, in northwestern Nigeria. They had been working on a bank construction project.
David Cameron, the British prime minister, said he had given the go-ahead for the joint Nigerian and British rescue operation after "credible information" had been received.
Both men were killed before their rescuers could reach them.
"We are still awaiting confirmation of the details, but the early indications are clear that both men were murdered by their captors, before they could be rescued," Cameron said.
He said the Nigerian authorities, with British support, had launched the attempt to rescue the men after "a window of opportunity arose to secure their release".
"Preparations were made to mount an operation to attempt to rescue Chris and Franco. Together with the Nigerian government, today I authorised it to go ahead, with UK support," he added.
"It is with great regret that I have to say that both Chris and Franco have lost their lives."
Cameron did not indicate if it was clear when the men were killed.
The two hostages, he said, had been held by "terrorists" who had made "very clear threats to take their lives", and the captives had been in "imminent and growing danger".
The AFP news agency first received a video showing McManus and Lamolinara in August. In the footage, both men said their kidnappers were from al-Qaeda. In a second video received by a Mauritanian news agency and seen by AFP in December, masked gunmen threatened to execute McManus if their demands were not met.
'Lots of speculation'
The Mauritanian agency claimed that the captors belonged to a Nigerian group inspired by al-Qaeda.
Cameron did not specify the identity of the kidnappers.
"It's not clear exactly who this group is," reported Al Jazeera's Yvonne Ndege from Lagos.
"There has been lots of speculation about the relationship between wings of al-Qaeda and terrorist organisations here in Nigeria, like the Boko Haram Islamist group who have been involved in acts of violence and kidnapping,
"Now there's nothing to suggest that these kidnappers were part of that sect, but clearly this is further heightening the tension and the fear that is felt in parts of this region about security issues in general."
The Italian government also confirmed that the two hostages, who had worked for the Italian construction company Stabilini Visinoni, had died.
McManus's family said in a statement they were "devastated" by the news.
"We knew Chris was in an extremely dangerous situation. However, we knew that everything that could be done was being done," they said in a statement.
They also expressed their condolences to Lamolinara's family.
Al Jazeera's Ndege said the security situation in northern Nigeria was deteriorating.
"The Nigerian authorities have been doing their best to try and get the security situation under control. They have deployed lots of police and military in many of the areas," she said.
She said much of the deployment in northern states was in places that had experienced attacks by Boko Haram.
"Obviously news of the kidnap and news of this failed rescue mission will only serve to make people ... a little bit more worried about how in control the security services are," she said.