A Nigerian Islamist group that killed seven foreign hostages it had been holding since February has posted a video of their bodies on the Internet.

Italian and Greek authorities confirmed on Sunday that a British, an Italian, a Greek and four Lebanese construction workers abducted in northern Nigeria last month had been killed by their captors.

British foreign secretary William Hague said it was "very likely" they were dead, calling it "an act of cold blooded murder." Nigerian authorities have thus far refused to confirm or deny the killings.

The silent video published by the al-Qaeda linked group dated March 9 shows a gunman standing next to a pile of bodies, then a series of close-ups of their faces lit up by a torch.

It is entitled in Arabic "The Killing of the seven Christian hostages in Nigeria." A caption underneath says in Arabic and in English: "In the name of Allah Most Beneficient Most Merciful".

The workers were seized from the premises of Lebanese construction firm Setraco in the remote town of Jama'are in the northern state of Bauchi.

About a week after they were taken, the group claimed responsibility and said it was because of "atrocities done to the religion of Allah by the European countries in many places such such as Afghanistan and Mali".  The British foreign office named the British hostage as Brendan Vaughan. An intelligence source in the Nigerian capital Abuja named the Italian as Silvano Trevisan, adding that he had been suffering from hypertension and heart problems.

The group killed a British and Italian hostage in northwest Nigeria during a failed rescue mission by British and Nigerian forces a year ago. Italy and Greece both said there had been no attempted rescue of the foreign hostages this time around, although Britain has not commented.

Attacks by Islamist groups in northern Nigeria have become the biggest threat to the stability of Africa's top oil producer. Western governments are concerned Nigerian Islamists have forged growing ties with groups elsewhere in the region, including al Qaeda's North African wing.

French intervention to flush Islamist groups out of northern Mali has greatly increased the risk posed by Islamists to Western interests in countries north and west Africa -- a risk highlighted by the dramatic attack on an Algerian gas plant in January in which at least 37 foreigners were killed.

Ansaru declared itself a separate group from Boko Haram in January, although security officials believe them to be closely linked. Its full name is Jama'atu Ansarul Musilimina Fi Biladis Sudan ("Vanguards for the Protection of Muslims in Black Africa").