An Islamist terror group in Nigeria is believed to have killed seven foreign hostages, including a Briton, an Italian, a Greek and up to four Lebanese.

The killings mark one of the worst acts of violence against expatriates in the country for decades.

Ansaru, a little-known but extreme group based in the north of Nigeria, said on Saturday it had killed the hostages, all employees of a Lebanese-owned building company, Setraco. The workers were kidnapped in Bauchi state, in the volatile northeast region of Nigeria, three weeks ago.

The group on Monday posted a video titled in Arabic “The killing of the seven Christian hostages in Nigeria”, Reuters reported. The silent video was dated March 9 and showed a gunman standing next to a pile of corpses, followed by close-ups of the faces, the news agency said.

British foreign secretary William Hague said on Sunday the government believed that all the hostages were dead, including a Briton named as Brendan Vaughan.

“It is with deep sadness that I must confirm that a British construction worker, held hostage in Nigeria since 16 February, is likely to have been killed at the hands of his captors, along with six other foreign nationals who we believe were also tragically murdered,” Mr Hague said. “This was an act of cold-blooded murder, which I condemn in the strongest terms.”

The Italian foreign ministry said it believed that one of its citizens, named as Silvano Trevisan, had been killed in an act of “barbarous and blind violence”. Confirming the death of a Greek citizen, Greece’s foreign ministry said Ansaru had “never communicated or formulated demands” to release the hostages.

The other four victims are thought to be Lebanese, although some reports at the time of the abductions said they included two Syrians. Ansaru said in an online message that it had killed the hostages because of attempts by Nigerian and British forces to free them. Both Italy and Greece denied any rescue attempt had taken place. The Nigerian government said it had no information about the death of the hostages.

Most of the violence in northern Nigeria, particularly in the northeast, has been carried out by the Islamist sect Boko Haram, which has a mostly domestic agenda. Parts of the group are believed to have broken away to form new movements, including Ansaru, whose full name translates to “Vanguards for the Protection of Muslims in Black Africa”.

Ansaru emerged publicly last year, killing two building workers from Britain and Italy who had been taken hostage in northwest Nigeria in 2011. They were killed during a botched rescue attempt by Nigerian troops and British special forces. Britain later designated Ansaru as a terrorist organisation, saying it was “broadly aligned with al-Qaeda”.

The militant group also kidnapped a French engineer in Katsina in December and in January attacked Nigerian troops who were about to deploy to Mali as part of a regional force. When claiming responsibility for the abduction of the seven hostages last month, Ansaru said the kidnap was partly in response to European military intervention in Muslim countries such as Mali and Afghanistan.

Kidnapping of foreigners has been a lucrative industry in the oil rich delta region in southern Nigeria for years. Hostages were usually freed unharmed once ransoms were paid. But abductions in the north are a different matter, because the most common outcome has been death. A German building worker abducted in Kano a year ago by an al-Qaeda-linked group was also killed. Many companies operating in Nigeria now prevent any expatriate staff travelling to the north because of the security risk.

Seven members of a French family were still missing after being kidnapped in neighbouring Cameroon in February. Militants who said they were from Boko Haram claimed responsibility.

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