LONDON (AP) ― Two Europeans held hostage in Nigeria by kidnappers claiming ties to al-Qaida were killed before rescuers could free them, authorities said Thursday.

The men ― a Briton and an Italian ― were killed by their captors. A Nigerian official said the two died in the crossfire during the rescue attempt, Prime Minister David Cameron said.

Chris McManus and Franco Lamolinara were taken captive in May. The pair had been working on a bank construction project in Nigeria, a country that has seen a rise in violence linked to the Islamist extremist group Boko Haram.

Britain’s Foreign Office confirmed two men were held by terrorists associated with Boko Haram, and a senior British government official said the kidnappers appeared to be from an al-Qaida-linked cell within Boko Haram, but not within the group’s main faction.

The British prime minister said that after months of not knowing where the men were, U.K. authorities had “received credible information about their location.” Believing the men’s lives were in “imminent and growing danger,’’ a rescue operation was mounted, Cameron said.

“The terrorists holding the two hostages made very clear threats to take their lives, including in a video that was posted on the Internet,’’ Cameron said.

U.K. officials had asked journalists not to publicize the video, in which the kidnappers claimed they belonged to al-Qaida and threatened to kill McManus and Lamolinara if their demands were not met.