Algerian special forces are continuing to hunt Islamist militants who overran a BP gas plant as freed British hostages headed home.
As the stand-off at the remote desert facility entered a fourth day it was thought that around 10 British workers remained "at risk".
The Algerian state news agency APS reported on Friday night a "provisional" figure of 12 foreign and Algerian workers who had been killed in the fighting at the plant at In Amenas.
It said about 100 foreign workers had been released from a total of 132 seized by the militants had been freed - along with 573 local employees. Eighteen of the militants were also reported to have been killed - suggesting that around a dozen of the original group of about 30 could still be at large.
A Mauritanian news site reported that the remaining militants were demanding the release of two terrorists held in the United States, including 1993 World Trade Centre bombing mastermind Omar Abdel Rahman, in return for the release of two US captives.
Despite the casualties among the hostages, an Algerian government source quoted by APS strongly defended the military operation, saying it had prevented a "true disaster" which would have caused "immeasurable" human and material damage.
The rescue mission had been carried out in "extremely complex circumstances" against terrorists armed with a huge arsenal of missiles, rocket launchers, grenades, and assault rifles, the source said, adding that it was the "only way to minimise or neutralise the deadly intent of the multinational terrorists - but not without the inevitable risks in all such situations".
Prime Minister David Cameron said Britain would do "everything we can to hunt down" those behind the "brutal and savage" attack. But he publicly expressed his frustration that he had not been notified in advance of the special forces assault.
He said that he had offered UK "technical and intelligence support" - including from experts in hostage negotiation and rescue - but that it was not taken up.
It was reported that two UK workers were among the casualties, which would bring the British death toll to three after one was killed in Wednesday's initial raid by the militants.
