Algerian officials say they may have tried to escape into the Sahara desert and got lost. It is not known whether any of them were British.
The hostage crisis at the In Amenas facility left 37 foreigners of eight nationalities, including two Scots, dead.
A spokesman for Algerian Prime Minister Abdemalek Sellal said of the missing foreigners: "Are they dead? Did they attempt to flee the site after the attack like some other expatriates? Are they lost in the desert after taking a wrong turn? These are all questions we ask ourselves."
The desert in the area is flat, rocky and featureless. And while roasting hot in the summer, during the winter months the temperature drops to 3C at night.
Foreign Secretary William Hague updated the Cabinet on the situation but had no new information on the fates of missing Britons.
Work is continuing to establish the identities of a number of bodies found in the gas compound.
Mr Cameron's official spokesman told a regular Westminster media briefing: "We will be continuing to co-operate with the Algerian authorities as they look into the causes behind the terrorist outrage and as the international community seeks to bring those who perpetrated it to justice."
Meanwhile, Canada's foreign minister has said he is trying to verify whether two Canadians were among the Islamists involved in the Algerian siege.
Algeria's Prime Minister said the hostage raid at a remote desert gas plant was co-ordinated by a Canadian.
Mr Sellal said the Canadian organising the attack, named only as Chedad, was among 29 militants killed.