An Italian and a British hostage kidnapped in May in Nigeria were killed on Thursday by their captors during a joint raid by British and Nigerian forces trying to free them, Italy's government said.

British Prime Minister David Cameron called Italian Prime Minister Mario Monti to inform him of the "tragic conclusion" of the operation, a statement said.

The joint forces intervened to free Italian Franco Lamolinara and Briton Christopher McManus, fearing that their lives were under threat, the statement said.

Cameron said it appeared McManus and Lamolinara had been "murdered by their captors before they could be rescued," the BBC reported.

A witness told Reuters that security forces had tried to force their way into a compound in Sokoto, northwest Nigeria.

"The security agencies tried to break into the house but there was resistance. The people inside the house were shooting at them and they returned fire. They exchanged fire for some time," said Mahmoud Abubakar, who lives on the same street.

"I saw a military truck come out of the compound with two bodies on it. I didn't see their color, because they were covered with leaves," he added.

The captors were a faction of militant Islamist sect Boko Haram, a senior official at Nigeria's State Security Service said.

"The hostage-takers shot the hostages before they even entered the compound. All the terrorists have been killed as well," he said. "We arrested some suspects a few days before who led us to them."

British special forces were involved in the rescue, UK media reports said.

McManus’ family said they were devastated by the news.

“We knew Chris was in an extremely dangerous situation,” the family said in a statement published on The Telegraph. “However we knew that everything that could be done was being done. Our thoughts are also of course with the loved ones of Chris’ colleague, Franco Lamolinara, who are also coming to terms with this truly sad news.”

McManus and Lamolinara were working as engineers for a large construction company called Stabilini Visinoni Limited when they were kidnapped on May 12, 2011 in in Birnin Kebbi city, The Telegraph reported.

Mcmanus Family / AFP - Getty Images

Chris McManus in an undated family photo.

Kidnappings have become increasingly common in Nigeria in recent years.

Two Britons were held by the Movement for the Emancipation of Niger Delta in September 2008, the newspaper reported. A Scottish oil worker was abducted and his guard killed in April 2009, according to the Telegraph.

Cameron, in a statement published on the Telegraph, said the rescue attempt went forward after authorities “received credible information about their location.”

“A window of opportunity arose to secure their release, " Cameron said. "We also had reason to believe that their lives were under imminent and growing danger."

It was not immediately clear how the two men were killed.

In August a video of the hostages surfaced in the Nigerian capital Abuja with the two men on their knees and blindfolded, with three men wearing turbans and holding guns and ammunition behind them.

This article includes reporting by Reuters and msnbc.com staff.

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