The wife of one of the men guarding the victims described the executions amid an escalating diplomatic row between Italy and Britain over the unsuccessful rescue mission.
Italian officials, including the country’s president, have said it is inexplicable that Britain did not inform its European partner before launching the attempt to free the two construction workers.
Chris McManus and Italian Franco Lamolinara were kidnapped last May while working in north-west Nigeria and killed on 8 March.
The guard’s wife – who gave her name as Hauwa – said bullets flew into the room where she and her husband were staying at the time of the rescue bid, killing her husband.
“After that, there were about six men who came out of the house with the two hostages,” she said. “They came into our wing of the compound, pushed the captives into the toilet and just shot them. I screamed.”
She denied knowing the hostages had been living in the same compound as her and said they had been kept in the main house which she was strictly forbidden to enter.
Nigerian authorities have detained five Islamist militants suspected of involvement in the kidnapping. Two of the men were arrested before the rescue attempt and three at the compound where the raid took place.
Italy’s foreign minister, Giulio Terzi, has asked Britain to provide the “utmost clarity” around the events, and demanded details urgently.
William Hague, the British foreign secretary, said the limited time available to make a decision “constrained how much we were able to consult others”.
But Downing Street has insisted that a possible rescue attempt had been raised with the Italian government beforehand and no objections were raised.
McManus, 28, from Oldham, Greater Manchester, and Lamolinara are believed to have been kidnapped by a faction of the militant Islamist sect Boko Haram.
The Italian and British governments tried to play down tensions on Friday in a joint statement issued after a meeting between Hague and Terzi. Hague had “made clear that there had been a limited opportunity to secure the release of the two hostages, whose lives were in imminent and growing danger”, they said. The statement added: “Under these circumstances it was only possible to inform Italy once the operation was under way.
“Mr Terzi expressed deep sorrow and disappointment over the tragic outcome of the operation and both ministers agreed on the urgency of sharing full information to facilitate the reconstruction and understanding of these events.
“Both ministers conveyed their solidarity and sympathy for the families of Chris McManus and Franco Lamolinara.”
Nigerian and British forces are reported to have fought for at least an hour in daylight after capturing a gang member who told them the hostages were being held in a disused house in Sokoto, north-west Nigeria.
British government officials in Nigeria then asked for immediate permission to launch a raid because of fears that the hostage-takers had discovered that their whereabouts were known to the authorities, sources said.
British special forces mounted the rescue attempt as soon as intelligence revealed where the two hostages were being held and – crucially – that the hostages were about to be moved, senior Whitehall officials briefed on the operation said on Friday night.
“It was not a matter of if they were going to be killed but when,” said one official, reflecting a unapologetic mood within Whitehall about the decision.
Nevertheless, British and Nigerian intelligence agencies and special forces are under pressure to explain exactly what went wrong and why – not least to the angry Italians. In the UK, senior ministers will be demanding a full debriefing on the incident.