A video posted online appears to show the bodies of some of the foreign hostages seized by a Nigerian Islamist militant group last month.
The silent video shows a gunman standing next to a pile of bodies, reports say. It also carries captions attributed to militant group Ansaru.
In an earlier online statement, Ansaru said it had killed the seven captives.
But Nigerian Interior Minister Aba Moro told BBC Hausa there was no confirmation the hostages were dead.
"As long as it remains unconfirmed, efforts will be made to ensure their rescue and security," he said on Monday.
A Briton, an Italian, a Greek and four Lebanese workers were captured in a raid on the Setraco construction site in the northern state of Bauchi on 16 February. One security guard died during the assault.
Ansaru, a suspected offshoot of the Boko Haram network, said it had carried out the attack in revenge for what it called atrocities by European nations against Islam.
On Monday, the militant group purportedly posted a video online with the caption: "The killing of seven Christian hostages in Nigeria."
In it, a gunman with a rifle is seen standing in the sand next to several bodies, the Associated Press reports.
The video appears to match grainy images of bodies posted online by Ansaru on Saturday.
In an accompanying online statement, the militant group said it had killed the captives.
On Sunday, UK Foreign Secretary William Hague said a British construction worker from Leeds, named as Brendan Vaughan, was "likely to have been killed at the hands of his captors, along with six other foreign nationals".
"This is an unforgivable act of pure, cold-blooded murder, for which there can be no excuse or justification," he added.
Italy also said it believed the seven were dead, while the Greek foreign ministry said: "The information we have shows that the Greek citizen is dead."
Ansaru said the hostages had been killed because of a rescue attempt by British and Nigerian forces.
But the Italian foreign ministry said there "was never any military attempt to rescue the hostages by any of the governments concerned".
It added that the group's actions "can have no explanation other than blind and barbaric violence".
Meanwhile Britain military said its warplanes which were recently spotted in Nigeria's capital Abuja had been there to carry soldiers taking part in the French-led operation in Mali - not to rescue hostages.