Five Islamist militants were reportedly captured alive by Algerian special forces during the violent end to the desert hostage crisis.
This brings the number of terrorists involved in the crisis to 37.
Algeria said it expected to raise its preliminary estimates of the number oif people killed in the four-day Saharan siege.
Bomb squads searching the gas plant where Islamist militants took foreign workers hostage say they have found "numerous" new bodies.
An official said the bodies were badly disfigured and difficult to identify. "The bodies could be either Algerian or foreign hostages," he added.
Three Britons have been killed and three more are believed dead after the Algeria hostage crisis.
Speaking at Chequers, Prime Minister David Cameron said one further UK resident is also thought to be dead.
An unknown number of Britons have survived. Some returned to the UK overnight, the Foreign Office said.
A raid by Algerian troops ended a four-day siege at the In Amenas gas facility on Saturday 19 January. At least 23 hostages died and Algeria says 32 militants were killed.
Mr Cameron said the priority now was to get the survivors home from Algeria.
He said: "I know the whole country will want to join me in sending our sympathies and condolences to the families who have undergone an absolutely dreadful ordeal and who now face life without these very precious loved ones."
The prime minister said he would be asking questions about the Algerian response to the crisis, but he added: "The responsibility for these deaths lies squarely with the terrorists who launched this vicious and cowardly attack." Cameron also sounded a cautionary note, warning of "decades of terrorism struggle ahead".
The storming of the facility by the Algerian army killed seven hostages, increasing the death toll among the captives to 23. All 32 heavily armed militants, who were believed to have had strong links to the Islamic militant group al-Qaida, were also killed.
Unconfirmed reports suggest the captors killed the hostages at the last minute when the army was trying to re-take the gas plant.
Officials have revealed details about the Algerian army operation. Although the hostages included foreign nationals, their governments were largely kept in the dark.
The authorities have provided a list of deadly weapons recovered from the terrorists slain at the natural gas plant.
The list includes "six machine guns (FMPK), 21 PMAK rifles, two shotguns, two 60mm mortars with rockets, six missiles of type C5 60mm with launchers, two RPG7 with eight rockets, 10 grenades arranged in explosive belts".
Algerian forces have also picked up "foreign military uniforms and a stock of ammunition and explosives". The weapons, which were thought to have been seized from the gas plant, were shown on state television.
The Algerian interior ministry has said 685 local workers and 107 foreigners were rescued from the siege.
A security source told state-run APS news: "To avoid a bloody turn of events and with the extremely dangerous situation, due to the clear intention (of terrorists) to flee the country with the hostages and blow out the gas facilities, the ANP Special Forces launched, with operational efficiency and professionalism, a targeted intervention to neutralise the terrorist group."
French President Francois Hollande has openly defended the Algerian army's response saying it was the "most suitable".
"When you have people taken hostage in such large numbers by terrorists with such cold determination and ready to kill those hostages - as they did - Algeria has an approach which to me, as I see it, is the most appropriate because there could be no negotiation," said Hollande.
Other western nations have reacted sharply to the attack and vowed to take necessary steps.
Prime Minister Cameron has confirmed that one Briton has been killed in the attack and added: "We now fear the worst for the lives of five others who are not yet accounted for."
Cameron added: "There is no justification for taking innocent life in this way. Our determination is stronger than ever to work with allies right around the world to root out and defeat this terrorist scourge and those who encourage it".