RIYADH/LAGOS: A Nigerian Islamist group said on Saturday it had killed seven foreign hostages seized last month from a construction firm's compound in northern Nigeria, where Islamist insurgents have killed hundreds over the past two years.

But Nigerian authorities said they had no information on any such killing, and doubted the veracity of the statement.

In a statement released on an Islamist website, the Ansaru group said it had killed the hostages in response to attempts by Britain and Nigeria to free them, SITE Monitoring Service reported.

Ansaru is one of several Islamist groups that have become the main security threat in Nigeria, Africa's largest oil producer. The al Qaeda-aligned group blasted into the compound of Setraco, a Lebanese construction company, on Feb. 7, abducting a Briton, an Italian, a Greek and four Lebanese workers.

The statement issued in Arabic and English on an affiliate of the Sinam al-Islam network was accompanied by screen shots of a video purporting to show the dead hostages, SITE said.

One shot showed a man with gun standing above several figures lying on the ground. The image was not clear enough to see if they were dead or much detail about them.

The hostage-taking, in the remote town of Jama'are in Bauchi state, was the biggest number of foreigners seized in the mostly Muslim north since the Islamist insurgency there intensified two years ago.

"As far as I'm concerned, and to the best of my knowledge, nothing like that has happened," Bauchi Police Commissioner Mohammed Ladan told Reuters when asked about the online statement.

An intelligence official in the north also said he doubted the report, although he said some suspects linked to the kidnapping had been arrested last week.

Lebanese officials said they were checking the reports. Italy's foreign ministry also said it was checking. Britain did not officially comment.

Mohammed Abdullah, a spokesman for Setraco, also said he had heard nothing about any harm done to the hostages.

The group's full name is Jama'atu Ansarul Musilimina Fi Biladis Sudan, which roughly translates as "vanguards for the protection of Muslims in Black Africa".

Ansaru, believed to be an offshoot of the larger Boko Haram group, said it had decided to kill the hostages because of attempts by Britain and Nigeria to rescue them.

"(We) announced the capture of seven Christians foreigners and warned that should there be any attempt by force to rescue them will render their lives in danger," the statement said.

"The Nigeria and British government operation lead to the death of all the seven Christians foreigners," it said.

Ansaru was suspected of being behind the killing of a British and Italian hostage a year ago in northwest Nigeria and Britain's parliament has labelled it a terrorist organisation.

It also claimed responsibility for the kidnapping in December of a French national.

After abducting the seven foreigners in February, it said its actions were "based on the transgression and atrocities done to the religion of Allah by the European countries in many places such as Afghanistan and Mali".

Western governments are concerned the Islamists have linked up with groups elsewhere in the region, including al Qaeda's North African wing AQIM.

France intervened in Mali last month as Islamist forces, which hijacked a rebellion by ethnic Touaregs to seize control of the north following a military coup in March 2012, pushed south towards the capital Bamako.

Kidnapping of foreigners for ransom has been common in Nigeria's southern oil region for a decade but abductions by radical Islamists in the north only began two years ago.