A British and an Italian hostages have been killed by their captors in Nigeria, after a failed hostage 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 and Franco Lamolinara, 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.

Joint operation

"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".