Three Britons have been killed and another three are believed to have died in the Algerian hostage crisis, David Cameron has announced.
The Prime Minister said a British resident is also thought to have been killed in the four day stand-off in the desert which dramatically ended on Saturday.
Mr Cameron called the deaths at the In Amenas gas plant an "appalling terrorist incident" and said every effort was being made to help surviving hostages come home.
The death toll includes the Briton already known to have died on Wednesday in the initial raid staged by a band of Islamist militants.
Mr Cameron said: "We now know that three British nationals have been killed, and a further three are believed to be dead. A further British resident is also believed to be dead.
Freed British hostages Peter (left) and Alan (right). No surnames available
"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 now face life without these very precious loved ones."
He continued: "The priority now must be to get everybody home from Algeria."
Mr Cameron said the attack was a "stark reminder" of the continuing terrorist threat and vowed to use Britain's chairmanship of the G8 to ensure that it was at the top of the international agenda.
"This is a global threat and it will require a global response. It will require a response that is about years, even decades, rather than months," he said.
"It requires a response that is patient and painstaking, that is tough but also intelligent, but above all has an absolutely iron resolve and that is what we will deliver over these coming years."
Algerian police escort freed Norwegian hostage Oddvar Birkedal
So far, in total, 23 hostages and 32 terrorists are known to have died with 107 foreign workers and 685 local employees released.
Algeria's chief government spokesman said on Sunday that he "strongly feared" the final hostage death toll would be higher.
According to Reuters, Algerian media are reporting that 25 bodies have so far been found inside the plant.
Scotland's First Minister Alex Salmond said two Scots, or people with immediate family connections in Scotland, are believed to have been killed.
Carlos Estrada, a Colombian man who lived in London with his family and worked for BP, has been confirmed dead by President Juan Manuel Santos.
At least one American died before Saturday's assault, and Japanese engineering firm JGC Corp said 10 of its Japanese and seven of its foreign workers are still unaccounted for.
Despite the major loss of life, Mr Cameron refused to criticise Algeria's handling of the crisis.
"The responsibility for these deaths lies squarely with the terrorists who launched these vicious and cowardly attacks," he said.
"When you are dealing with a terrorist incident on this scale with up to 30 terrorists it is extremely difficult to respond and get this right in every respect."
An Algerian army truck in a street of In Amenas, near the gas plant
He added: "What we face is an extremist, Islamist, al Qaeda-linked terrorist group. Just as we had to deal with that in Pakistan and in Afghanistan so the world needs to come together to deal with this threat in north Africa.
"It is linked to al Qaeda, it wants to destroy our way of life, it believes in killing as many people as it can.
"We need to work with others to defeat the terrorists and to close down the ungoverned spaces where they thrive with all the means that we have."
Foreign Secretary William Hague said 22 British nationals had survived the crisis and were already back in the UK.
Britain's ambassador is currently in Algiers and will travel to the south of the country today to help any other Britons trying to leave.
He told Sky's Murnaghan show: "These are cold-blooded murderers and we must remember that. There is no political excuse that justifies behaving in this way."
He also defended Algeria's response, revealing that he had been told the terrorists had been planning to blow up the whole installation - causing even more deaths.
The drama started on Wednesday when a group of around 30 heavily-armed militants mounted a dawn raid on the plant which is close to the Libyan border.
They seized hostages from among the 700 Algerian and foreign workers at the site. Two members of staff, including one Briton, died in the initial assault.
Algerian special forces mounted an operation to take back the plant 24 hours later, without informing any other governments in advance.
The Algerians insisted later that they had to act due to fears that the militants were about to flee into the desert, taking hostages with them.
Algerian police guard the entrance of a hospital located near the gas plant
Fears for the hostages' safety grew amid reports of fierce fighting and multiple deaths.
By Friday, it was being claimed that around 100 foreigners - from a total of 132 - and 573 Algerians had been freed.
Provisional figures put the death toll at 12 hostages and 18 militants but it was clear one group of terrorists was still holding out.
On Saturday, Algerian troops launched a final assault on the site and brought the stand-off to a bloody conclusion.
The state news agency, APS, reported that the terrorists had executed seven of the remaining hostages before they themselves were killed.
Algerian security forces escort a bus carrying freed hostages
Troops later found an arsenal of six machine guns, 21 rifles, two shotguns, two 60mm mortars with shells, six 60mm missiles with launchers, two rocket-propelled grenades with eight rockets and 10 grenades in explosive belts.
The terrorists also booby-trapped the sprawling plant with explosives before the last shoot-out.
Algeria's interior ministry has strongly defended the rescue operation.
"To avoid a bloody turn of events in response to the extreme danger of the situation, the army's special forces launched an intervention with efficiency and professionalism to neutralise the terrorist groups that were first trying to flee with the hostages and then blow up the gas facilities," it said in a statement.
The kidnappers are part of the Masked Brigade - a terrorist splinter group led by the veteran jihadist, Mokhtar Belmokhtar, which broke away from al Qaeda in the Islamic Maghreb.
They initially claimed the attack was in retaliation for the French military intervention in neighbouring Mali.
The gas plant is in the desert in In Amenas
It was subsequently reported that they were demanding the release of two terrorists held in the US, including 1993 World Trade Centre bombing mastermind Omar Abdel Rahman, in return for the release of two US captives.
Algeria has fought its own Islamist rebellion since the 1990s, elements of which later declared allegiance to al Qaeda and then set up new groups in the poorly patrolled wastes of the Sahara, where they flourished.
The plant at In Amenas is jointly operated by BP, Norwegian company Statoil and Algerian state oil company Sonatrach.
As freed hostages began to leave the plant, accounts emerged of their horrific treatment at the hands of the kidnappers.
One Algerian worker, who gave his name only as Chabane, described how he heard the militants talking with Libyan, Egyptian and Tunisian accents.
At one point, he said, they caught a Briton.
"They threatened him until he called out in English to his friends, telling them 'Come out, come out. They're not going to kill you. They're looking for the Americans'," he said. "A few minutes later they blew him away."
Paul Morgan 
Another image shown on Algerian TV 
The trains crashed in the Penzing district of western Vienna 
A protester carries a gun as he joins Gun Appreciation Day 
Americans staunchly defend their right to bear arms 
The attacker was later escorted from the conference by security guards 
Mr Obama with his family at his first inauguration four years ago 
Armstrong competing before he was sanctioned 
The full interview will be shown on Discovery at 8pm on Saturday 
