Two hostages held in Nigeria have been killed in a rescue attempt by British and Nigerian forces.

British Prime Minister David Cameron and Nigerian President Goodluck Jonathan jointly announced the hostages were killed by their captors before they could be rescued.

President Jonathan blamed the Boko Haram militant group for the kidnappings, and said that all the kidnappers have been arrested, without specifying how many. The hostages were rescued after a reported hours-long gunfight at a house in the town of Sokoto.

Mr. Cameron said Thursday that he approved the joint operation between British and Nigerian forces after getting information about the hostages' location. He said the intelligence also indicated the men's lives were in “imminent and growing danger.”

The two hostages, Italian national Franco Lamolinara and British national Christopher McManus, were kidnapped in northern Nigeria in May of last year. They later appeared in an Internet video with armed gunmen who claimed to be with the al-Qaida terrorist network.

British and Nigerian officials say they believe the captors were from an al-Qaida-linked splinter cell of Boko Haram, rather than from the group's main leadership.

Kidnappings are not uncommon in Nigeria's delta region, where tensions over oil profits run high. But they are rare in northwestern Nigeria where Sokoto is located, near the border with Niger.